OK, let's crack this. We're given 645 and our task is to multiply it by 387. Essentially, we'll be adding 645 to itself 387 times.
Step 1: 0 + 645 = 645
Step 2: 645 + 645 = 1290
Step 3: 1290 + 645 = 1935
Step 4: 1935 + 645 = 2580
Step 5: 2580 + 645 = 3225
Step 6: 3225 + 645 = 3870
Step 7: 3870 + 645 = 4515
Step 8: 4515 + 645 = 5160
Step 9: 5160 + 645 = 5805
Step 10: 5805 + 645 = 6450
Step 11: 6450 + 645 = 7095
Step 12: 7095 + 645 = 7740
Step 13: 7740 + 645 = 8385
Step 14: 8385 + 645 = 9030
Step 15: 9030 + 645 = 9675
Step 16: 9675 + 645 = 10320
Step 17: 10320 + 645 = 10965
Step 18: 10965 + 645 = 11610
Step 19: 11610 + 645 = 12255
Step 20: 12255 + 645 = 12900
Step 21: 12900 + 645 = 13545
Step 22: 13545 + 645 = 14190
Step 23: 14190 + 645 = 14835
Step 24: 14835 + 645 = 15480
Step 25: 15480 + 645 = 16125
Step 26: 16125 + 645 = 16770
Step 27: 16770 + 645 = 17415
Step 28: 17415 + 645 = 18060
Step 29: 18060 + 645 = 18705
Step 30: 18705 + 645 = 19350
Step 31: 19350 + 645 = 19995
Step 32: 19995 + 645 = 20640
Step 33: 20640 + 645 = 21285
Step 34: 21285 + 645 = 21930
Step 35: 21930 + 645 = 22575
Step 36: 22575 + 645 = 23220
Step 37: 23220 + 645 = 23865
Step 38: 23865 + 645 = 24510
Step 39: 24510 + 645 = 25155
Step 40: 25155 + 645 = 25800
Step 41: 25800 + 645 = 26445
Step 42: 26445 + 645 = 27090
Step 43: 27090 + 645 = 27735
Step 44: 27735 + 645 = 28380
Step 45: 28380 + 645 = 29025
Step 46: 29025 + 645 = 29670
Step 47: 29670 + 645 = 30315
Step 48: 30315 + 645 = 30960
Step 49: 30960 + 645 = 31605
Step 50: 31605 + 645 = 32250
Step 51: 32250 + 645 = 32895
Step 52: 32895 + 645 = 33540
Step 53: 33540 + 645 = 34185
Step 54: 34185 + 645 = 34830
Step 55: 34830 + 645 = 35475
Step 56: 35475 + 645 = 36120
Step 57: 36120 + 645 = 36765
Step 58: 36765 + 645 = 37410
Step 59: 37410 + 645 = 38055
Step 60: 38055 + 645 = 38700
Step 61: 38700 + 645 = 39345
Step 62: 39345 + 645 = 39990
Step 63: 39990 + 645 = 40635
Step 64: 40635 + 645 = 41280
Step 65: 41280 + 645 = 41925
Step 66: 41925 + 645 = 42570
Step 67: 42570 + 645 = 43215
Step 68: 43215 + 645 = 43860
Step 69: 43860 + 645 = 44505
Step 70: 44505 + 645 = 45150
Step 71: 45150 + 645 = 45795
Step 72: 45795 + 645 = 46440
Step 73: 46440 + 645 = 47085
Step 74: 47085 + 645 = 47730
Step 75: 47730 + 645 = 48375
Step 76: 48375 + 645 = 49020
Step 77: 49020 + 645 = 49665
Step 78: 49665 + 645 = 50310
Step 79: 50310 + 645 = 50955
Step 80: 50955 + 645 = 51600
Step 81: 51600 + 645 = 52245
Step 82: 52245 + 645 = 52890
Step 83: 52890 + 645 = 53535
Step 84: 53535 + 645 = 54180
Step 85: 54180 + 645 = 54825
Step 86: 54825 + 645 = 55470
Step 87: 55470 + 645 = 56115
Step 88: 56115 + 645 = 56760
Step 89: 56760 + 645 = 57405
Step 90: 57405 + 645 = 58050
Step 91: 58050 + 645 = 58695
Step 92: 58695 + 645 = 59340
Step 93: 59340 + 645 = 59985
Step 94: 59985 + 645 = 60630
Step 95: 60630 + 645 = 61275
Step 96: 61275 + 645 = 61920
Step 97: 61920 + 645 = 62565
Step 98: 62565 + 645 = 63210
Step 99: 63210 + 645 = 63855
Step 100: 63855 + 645 = 64500
Step 101: 64500 + 645 = 65145
Step 102: 65145 + 645 = 65790
Step 103: 65790 + 645 = 66435
Step 104: 66435 + 645 = 67080
Step 105: 67080 + 645 = 67725
Step 106: 67725 + 645 = 68370
Step 107: 68370 + 645 = 69015
Step 108: 69015 + 645 = 69660
Step 109: 69660 + 645 = 70305
Step 110: 70305 + 645 = 70950
Step 111: 70950 + 645 = 71595
Step 112: 71595 + 645 = 72240
Step 113: 72240 + 645 = 72885
Step 114: 72885 + 645 = 73530
Step 115: 73530 + 645 = 74175
Step 116: 74175 + 645 = 74820
Step 117: 74820 + 645 = 75465
Step 118: 75465 + 645 = 76110
Step 119: 76110 + 645 = 76755
Step 120: 76755 + 645 = 77400
Step 121: 77400 + 645 = 78045
Step 122: 78045 + 645 = 78690
Step 123: 78690 + 645 = 79335
Step 124: 79335 + 645 = 79980
Step 125: 79980 + 645 = 80625
Step 126: 80625 + 645 = 81270
Step 127: 81270 + 645 = 81915
Step 128: 81915 + 645 = 82560
Step 129: 82560 + 645 = 83205
Step 130: 83205 + 645 = 83850
Step 131: 83850 + 645 = 84495
Step 132: 84495 + 645 = 85140
Step 133: 85140 + 645 = 85785
Step 134: 85785 + 645 = 86430
Step 135: 86430 + 645 = 87075
Step 136: 87075 + 645 = 87720
Step 137: 87720 + 645 = 88365
Step 138: 88365 + 645 = 89010
Step 139: 89010 + 645 = 89655
Step 140: 89655 + 645 = 90300
Step 141: 90300 + 645 = 90945
Step 142: 90945 + 645 = 91590
Step 143: 91590 + 645 = 92235
Step 144: 92235 + 645 = 92880
Step 145: 92880 + 645 = 93525
Step 146: 93525 + 645 = 94170
Step 147: 94170 + 645 = 94815
Step 148: 94815 + 645 = 95460
Step 149: 95460 + 645 = 96105
Step 150: 96105 + 645 = 96750
Step 151: 96750 + 645 = 97395
Step 152: 97395 + 645 = 98040
Step 153: 98040 + 645 = 98685
Step 154: 98685 + 645 = 99330
Step 155: 99330 + 645 = 99975
Step 156: 99975 + 645 = 100620
Step 157: 100620 + 645 = 101265
Step 158: 101265 + 645 = 101910
Step 159: 101910 + 645 = 102555
Step 160: 102555 + 645 = 103200
Step 161: 103200 + 645 = 103845
Step 162: 103845 + 645 = 104490
Step 163: 104490 + 645 = 105135
Step 164: 105135 + 645 = 105780
Step 165: 105780 + 645 = 106425
Step 166: 106425 + 645 = 107070
Step 167: 107070 + 645 = 107715
Step 168: 107715 + 645 = 108360
Step 169: 108360 + 645 = 109005
Step 170: 109005 + 645 = 109650
Step 171: 109650 + 645 = 110295
Step 172: 110295 + 645 = 110940
Step 173: 110940 + 645 = 111585
Step 174: 111585 + 645 = 112230
Step 175: 112230 + 645 = 112875
Step 176: 112875 + 645 = 113520
Step 177: 113520 + 645 = 114165
Step 178: 114165 + 645 = 114810
Step 179: 114810 + 645 = 115455
Step 180: 115455 + 645 = 116100
Step 181: 116100 + 645 = 116745
Step 182: 116745 + 645 = 117390
Step 183: 117390 + 645 = 118035
Step 184: 118035 + 645 = 118680
Step 185: 118680 + 645 = 119325
Step 186: 119325 + 645 = 119970
Step 187: 119970 + 645 = 120615
Step 188: 120615 + 645 = 121260
Step 189: 121260 + 645 = 121905
Step 190: 121905 + 645 = 122550
Step 191: 122550 + 645 = 123195
Step 192: 123195 + 645 = 123840
Step 193: 123840 + 645 = 124485
Step 194: 124485 + 645 = 125130
Step 195: 125130 + 645 = 125775
Step 196: 125775 + 645 = 126420
Step 197: 126420 + 645 = 127065
Step 198: 127065 + 645 = 127710
Step 199: 127710 + 645 = 128355
Step 200: 128355 + 645 = 129000
Step 201: 129000 + 645 = 129645
Step 202: 129645 + 645 = 130290
Step 203: 130290 + 645 = 130935
Step 204: 130935 + 645 = 131580
Step 205: 131580 + 645 = 132225
Step 206: 132225 + 645 = 132870
Step 207: 132870 + 645 = 133515
Step 208: 133515 + 645 = 134160
Step 209: 134160 + 645 = 134805
Step 210: 134805 + 645 = 135450
Step 211: 135450 + 645 = 136095
Step 212: 136095 + 645 = 136740
Step 213: 136740 + 645 = 137385
Step 214: 137385 + 645 = 138030
Step 215: 138030 + 645 = 138675
Step 216: 138675 + 645 = 139320
Step 217: 139320 + 645 = 139965
Step 218: 139965 + 645 = 140610
Step 219: 140610 + 645 = 141255
Step 220: 141255 + 645 = 141900
Step 221: 141900 + 645 = 142545
Step 222: 142545 + 645 = 143190
Step 223: 143190 + 645 = 143835
Step 224: 143835 + 645 = 144480
Step 225: 144480 + 645 = 145125
Step 226: 145125 + 645 = 145770
Step 227: 145770 + 645 = 146415
Step 228: 146415 + 645 = 147060
Step 229: 147060 + 645 = 147705
Step 230: 147705 + 645 = 148350
Step 231: 148350 + 645 = 148995
Step 232: 148995 + 645 = 149640
Step 233: 149640 + 645 = 150285
Step 234: 150285 + 645 = 150930
Step 235: 150930 + 645 = 151575
Step 236: 151575 + 645 = 152220
Step 237: 152220 + 645 = 152865
Step 238: 152865 + 645 = 153510
Step 239: 153510 + 645 = 154155
Step 240: 154155 + 645 = 154800
Step 241: 154800 + 645 = 155445
Step 242: 155445 + 645 = 156090
Step 243: 156090 + 645 = 156735
Step 244: 156735 + 645 = 157380
Step 245: 157380 + 645 = 158025
Step 246: 158025 + 645 = 158670
Step 247: 158670 + 645 = 159315
Step 248: 159315 + 645 = 159960
Step 249: 159960 + 645 = 160605
Step 250: 160605 + 645 = 161250
Step 251: 161250 + 645 = 161895
Step 252: 161895 + 645 = 162540
Step 253: 162540 + 645 = 163185
Step 254: 163185 + 645 = 163830
Step 255: 163830 + 645 = 164475
Step 256: 164475 + 645 = 165120
Step 257: 165120 + 645 = 165765
Step 258: 165765 + 645 = 166410
Step 259: 166410 + 645 = 167055
Step 260: 167055 + 645 = 167700
Step 261: 167700 + 645 = 168345
Step 262: 168345 + 645 = 168990
Step 263: 168990 + 645 = 169635
Step 264: 169635 + 645 = 170280
Step 265: 170280 + 645 = 170925
Step 266: 170925 + 645 = 171570
Step 267: 171570 + 645 = 172215
Step 268: 172215 + 645 = 172860
Step 269: 172860 + 645 = 173505
Step 270: 173505 + 645 = 174150
Step 271: 174150 + 645 = 174795
Step 272: 174795 + 645 = 175440
Step 273: 175440 + 645 = 176085
Step 274: 176085 + 645 = 176730
Step 275: 176730 + 645 = 177375
Step 276: 177375 + 645 = 178020
Step 277: 178020 + 645 = 178665
Step 278: 178665 + 645 = 179310
Step 279: 179310 + 645 = 179955
Step 280: 179955 + 645 = 180600
Step 281: 180600 + 645 = 181245
Step 282: 181245 + 645 = 181890
Step 283: 181890 + 645 = 182535
Step 284: 182535 + 645 = 183180
Step 285: 183180 + 645 = 183825
Step 286: 183825 + 645 = 184470
Step 287: 184470 + 645 = 185115
Step 288: 185115 + 645 = 185760
Step 289: 185760 + 645 = 186405
Step 290: 186405 + 645 = 187050
Step 291: 187050 + 645 = 187695
Step 292: 187695 + 645 = 188340
Step 293: 188340 + 645 = 188985
Step 294: 188985 + 645 = 189630
Step 295: 189630 + 645 = 190275
Step 296: 190275 + 645 = 190920
Step 297: 190920 + 645 = 191565
Step 298: 191565 + 645 = 192210
Step 299: 192210 + 645 = 192855
Step 300: 192855 + 645 = 193500
Step 301: 193500 + 645 = 194145
Step 302: 194145 + 645 = 194790
Step 303: 194790 + 645 = 195435
Step 304: 195435 + 645 = 196080
Step 305: 196080 + 645 = 196725
Step 306: 196725 + 645 = 197370
Step 307: 197370 + 645 = 198015
Step 308: 198015 + 645 = 198660
Step 309: 198660 + 645 = 199305
Step 310: 199305 + 645 = 199950
Step 311: 199950 + 645 = 200595
Step 312: 200595 + 645 = 201240
Step 313: 201240 + 645 = 201885
Step 314: 201885 + 645 = 202530
Step 315: 202530 + 645 = 203175
Step 316: 203175 + 645 = 203820
Step 317: 203820 + 645 = 204465
Step 318: 204465 + 645 = 205110
Step 319: 205110 + 645 = 205755
Step 320: 205755 + 645 = 206400
Step 321: 206400 + 645 = 207045
Step 322: 207045 + 645 = 207690
Step 323: 207690 + 645 = 208335
Step 324: 208335 + 645 = 208980
Step 325: 208980 + 645 = 209625
Step 326: 209625 + 645 = 210270
Step 327: 210270 + 645 = 210915
Step 328: 210915 + 645 = 211560
Step 329: 211560 + 645 = 212205
Step 330: 212205 + 645 = 212850
Step 331: 212850 + 645 = 213495
Step 332: 213495 + 645 = 214140
Step 333: 214140 + 645 = 214785
Step 334: 214785 + 645 = 215430
Step 335: 215430 + 645 = 216075
Step 336: 216075 + 645 = 216720
Step 337: 216720 + 645 = 217365
Step 338: 217365 + 645 = 218010
Step 339: 218010 + 645 = 218655
Step 340: 218655 + 645 = 219300
Step 341: 219300 + 645 = 219945
Step 342: 219945 + 645 = 220590
Step 343: 220590 + 645 = 221235
Step 344: 221235 + 645 = 221880
Step 345: 221880 + 645 = 222525
Step 346: 222525 + 645 = 223170
Step 347: 223170 + 645 = 223815
Step 348: 223815 + 645 = 224460
Step 349: 224460 + 645 = 225105
Step 350: 225105 + 645 = 225750
Step 351: 225750 + 645 = 226395
Step 352: 226395 + 645 = 227040
Step 353: 227040 + 645 = 227685
Step 354: 227685 + 645 = 228330
Step 355: 228330 + 645 = 228975
Step 356: 228975 + 645 = 229620
Step 357: 229620 + 645 = 230265
Step 358: 230265 + 645 = 230910
Step 359: 230910 + 645 = 231555
Step 360: 231555 + 645 = 232200
Step 361: 232200 + 645 = 232845
Step 362: 232845 + 645 = 233490
Step 363: 233490 + 645 = 234135
Step 364: 234135 + 645 = 234780
Step 365: 234780 + 645 = 235425
Step 366: 235425 + 645 = 236070
Step 367: 236070 + 645 = 236715
Step 368: 236715 + 645 = 237360
Step 369: 237360 + 645 = 238005
Step 370: 238005 + 645 = 238650
Step 371: 238650 + 645 = 239295
Step 372: 239295 + 645 = 239940
Step 373: 239940 + 645 = 240585
Step 374: 240585 + 645 = 241230
Step 375: 241230 + 645 = 241875
Step 376: 241875 + 645 = 242520
Step 377: 242520 + 645 = 243165
Step 378: 243165 + 645 = 243810
Step 379: 243810 + 645 = 244455
Step 380: 244455 + 645 = 245100
Step 381: 245100 + 645 = 245745
Step 382: 245745 + 645 = 246390
Step 383: 246390 + 645 = 247035
Step 384: 247035 + 645 = 247680
Step 385: 247680 + 645 = 248325
Step 386: 248325 + 645 = 248970
Step 387: 248970 + 645 = 249615

So, 645*387 = 249615