Alright, let's solve this math problem step by step. First, we have the number 10. Next, we see the multiplication sign, which means we need to multiply something. And what are we multiplying it by? We multiply it by 399. So, we're going to take the number 10 and add it to itself 399 times.
Step 1: 0 + 10 = 10
Step 2: 10 + 10 = 20
Step 3: 20 + 10 = 30
Step 4: 30 + 10 = 40
Step 5: 40 + 10 = 50
Step 6: 50 + 10 = 60
Step 7: 60 + 10 = 70
Step 8: 70 + 10 = 80
Step 9: 80 + 10 = 90
Step 10: 90 + 10 = 100
Step 11: 100 + 10 = 110
Step 12: 110 + 10 = 120
Step 13: 120 + 10 = 130
Step 14: 130 + 10 = 140
Step 15: 140 + 10 = 150
Step 16: 150 + 10 = 160
Step 17: 160 + 10 = 170
Step 18: 170 + 10 = 180
Step 19: 180 + 10 = 190
Step 20: 190 + 10 = 200
Step 21: 200 + 10 = 210
Step 22: 210 + 10 = 220
Step 23: 220 + 10 = 230
Step 24: 230 + 10 = 240
Step 25: 240 + 10 = 250
Step 26: 250 + 10 = 260
Step 27: 260 + 10 = 270
Step 28: 270 + 10 = 280
Step 29: 280 + 10 = 290
Step 30: 290 + 10 = 300
Step 31: 300 + 10 = 310
Step 32: 310 + 10 = 320
Step 33: 320 + 10 = 330
Step 34: 330 + 10 = 340
Step 35: 340 + 10 = 350
Step 36: 350 + 10 = 360
Step 37: 360 + 10 = 370
Step 38: 370 + 10 = 380
Step 39: 380 + 10 = 390
Step 40: 390 + 10 = 400
Step 41: 400 + 10 = 410
Step 42: 410 + 10 = 420
Step 43: 420 + 10 = 430
Step 44: 430 + 10 = 440
Step 45: 440 + 10 = 450
Step 46: 450 + 10 = 460
Step 47: 460 + 10 = 470
Step 48: 470 + 10 = 480
Step 49: 480 + 10 = 490
Step 50: 490 + 10 = 500
Step 51: 500 + 10 = 510
Step 52: 510 + 10 = 520
Step 53: 520 + 10 = 530
Step 54: 530 + 10 = 540
Step 55: 540 + 10 = 550
Step 56: 550 + 10 = 560
Step 57: 560 + 10 = 570
Step 58: 570 + 10 = 580
Step 59: 580 + 10 = 590
Step 60: 590 + 10 = 600
Step 61: 600 + 10 = 610
Step 62: 610 + 10 = 620
Step 63: 620 + 10 = 630
Step 64: 630 + 10 = 640
Step 65: 640 + 10 = 650
Step 66: 650 + 10 = 660
Step 67: 660 + 10 = 670
Step 68: 670 + 10 = 680
Step 69: 680 + 10 = 690
Step 70: 690 + 10 = 700
Step 71: 700 + 10 = 710
Step 72: 710 + 10 = 720
Step 73: 720 + 10 = 730
Step 74: 730 + 10 = 740
Step 75: 740 + 10 = 750
Step 76: 750 + 10 = 760
Step 77: 760 + 10 = 770
Step 78: 770 + 10 = 780
Step 79: 780 + 10 = 790
Step 80: 790 + 10 = 800
Step 81: 800 + 10 = 810
Step 82: 810 + 10 = 820
Step 83: 820 + 10 = 830
Step 84: 830 + 10 = 840
Step 85: 840 + 10 = 850
Step 86: 850 + 10 = 860
Step 87: 860 + 10 = 870
Step 88: 870 + 10 = 880
Step 89: 880 + 10 = 890
Step 90: 890 + 10 = 900
Step 91: 900 + 10 = 910
Step 92: 910 + 10 = 920
Step 93: 920 + 10 = 930
Step 94: 930 + 10 = 940
Step 95: 940 + 10 = 950
Step 96: 950 + 10 = 960
Step 97: 960 + 10 = 970
Step 98: 970 + 10 = 980
Step 99: 980 + 10 = 990
Step 100: 990 + 10 = 1000
Step 101: 1000 + 10 = 1010
Step 102: 1010 + 10 = 1020
Step 103: 1020 + 10 = 1030
Step 104: 1030 + 10 = 1040
Step 105: 1040 + 10 = 1050
Step 106: 1050 + 10 = 1060
Step 107: 1060 + 10 = 1070
Step 108: 1070 + 10 = 1080
Step 109: 1080 + 10 = 1090
Step 110: 1090 + 10 = 1100
Step 111: 1100 + 10 = 1110
Step 112: 1110 + 10 = 1120
Step 113: 1120 + 10 = 1130
Step 114: 1130 + 10 = 1140
Step 115: 1140 + 10 = 1150
Step 116: 1150 + 10 = 1160
Step 117: 1160 + 10 = 1170
Step 118: 1170 + 10 = 1180
Step 119: 1180 + 10 = 1190
Step 120: 1190 + 10 = 1200
Step 121: 1200 + 10 = 1210
Step 122: 1210 + 10 = 1220
Step 123: 1220 + 10 = 1230
Step 124: 1230 + 10 = 1240
Step 125: 1240 + 10 = 1250
Step 126: 1250 + 10 = 1260
Step 127: 1260 + 10 = 1270
Step 128: 1270 + 10 = 1280
Step 129: 1280 + 10 = 1290
Step 130: 1290 + 10 = 1300
Step 131: 1300 + 10 = 1310
Step 132: 1310 + 10 = 1320
Step 133: 1320 + 10 = 1330
Step 134: 1330 + 10 = 1340
Step 135: 1340 + 10 = 1350
Step 136: 1350 + 10 = 1360
Step 137: 1360 + 10 = 1370
Step 138: 1370 + 10 = 1380
Step 139: 1380 + 10 = 1390
Step 140: 1390 + 10 = 1400
Step 141: 1400 + 10 = 1410
Step 142: 1410 + 10 = 1420
Step 143: 1420 + 10 = 1430
Step 144: 1430 + 10 = 1440
Step 145: 1440 + 10 = 1450
Step 146: 1450 + 10 = 1460
Step 147: 1460 + 10 = 1470
Step 148: 1470 + 10 = 1480
Step 149: 1480 + 10 = 1490
Step 150: 1490 + 10 = 1500
Step 151: 1500 + 10 = 1510
Step 152: 1510 + 10 = 1520
Step 153: 1520 + 10 = 1530
Step 154: 1530 + 10 = 1540
Step 155: 1540 + 10 = 1550
Step 156: 1550 + 10 = 1560
Step 157: 1560 + 10 = 1570
Step 158: 1570 + 10 = 1580
Step 159: 1580 + 10 = 1590
Step 160: 1590 + 10 = 1600
Step 161: 1600 + 10 = 1610
Step 162: 1610 + 10 = 1620
Step 163: 1620 + 10 = 1630
Step 164: 1630 + 10 = 1640
Step 165: 1640 + 10 = 1650
Step 166: 1650 + 10 = 1660
Step 167: 1660 + 10 = 1670
Step 168: 1670 + 10 = 1680
Step 169: 1680 + 10 = 1690
Step 170: 1690 + 10 = 1700
Step 171: 1700 + 10 = 1710
Step 172: 1710 + 10 = 1720
Step 173: 1720 + 10 = 1730
Step 174: 1730 + 10 = 1740
Step 175: 1740 + 10 = 1750
Step 176: 1750 + 10 = 1760
Step 177: 1760 + 10 = 1770
Step 178: 1770 + 10 = 1780
Step 179: 1780 + 10 = 1790
Step 180: 1790 + 10 = 1800
Step 181: 1800 + 10 = 1810
Step 182: 1810 + 10 = 1820
Step 183: 1820 + 10 = 1830
Step 184: 1830 + 10 = 1840
Step 185: 1840 + 10 = 1850
Step 186: 1850 + 10 = 1860
Step 187: 1860 + 10 = 1870
Step 188: 1870 + 10 = 1880
Step 189: 1880 + 10 = 1890
Step 190: 1890 + 10 = 1900
Step 191: 1900 + 10 = 1910
Step 192: 1910 + 10 = 1920
Step 193: 1920 + 10 = 1930
Step 194: 1930 + 10 = 1940
Step 195: 1940 + 10 = 1950
Step 196: 1950 + 10 = 1960
Step 197: 1960 + 10 = 1970
Step 198: 1970 + 10 = 1980
Step 199: 1980 + 10 = 1990
Step 200: 1990 + 10 = 2000
Step 201: 2000 + 10 = 2010
Step 202: 2010 + 10 = 2020
Step 203: 2020 + 10 = 2030
Step 204: 2030 + 10 = 2040
Step 205: 2040 + 10 = 2050
Step 206: 2050 + 10 = 2060
Step 207: 2060 + 10 = 2070
Step 208: 2070 + 10 = 2080
Step 209: 2080 + 10 = 2090
Step 210: 2090 + 10 = 2100
Step 211: 2100 + 10 = 2110
Step 212: 2110 + 10 = 2120
Step 213: 2120 + 10 = 2130
Step 214: 2130 + 10 = 2140
Step 215: 2140 + 10 = 2150
Step 216: 2150 + 10 = 2160
Step 217: 2160 + 10 = 2170
Step 218: 2170 + 10 = 2180
Step 219: 2180 + 10 = 2190
Step 220: 2190 + 10 = 2200
Step 221: 2200 + 10 = 2210
Step 222: 2210 + 10 = 2220
Step 223: 2220 + 10 = 2230
Step 224: 2230 + 10 = 2240
Step 225: 2240 + 10 = 2250
Step 226: 2250 + 10 = 2260
Step 227: 2260 + 10 = 2270
Step 228: 2270 + 10 = 2280
Step 229: 2280 + 10 = 2290
Step 230: 2290 + 10 = 2300
Step 231: 2300 + 10 = 2310
Step 232: 2310 + 10 = 2320
Step 233: 2320 + 10 = 2330
Step 234: 2330 + 10 = 2340
Step 235: 2340 + 10 = 2350
Step 236: 2350 + 10 = 2360
Step 237: 2360 + 10 = 2370
Step 238: 2370 + 10 = 2380
Step 239: 2380 + 10 = 2390
Step 240: 2390 + 10 = 2400
Step 241: 2400 + 10 = 2410
Step 242: 2410 + 10 = 2420
Step 243: 2420 + 10 = 2430
Step 244: 2430 + 10 = 2440
Step 245: 2440 + 10 = 2450
Step 246: 2450 + 10 = 2460
Step 247: 2460 + 10 = 2470
Step 248: 2470 + 10 = 2480
Step 249: 2480 + 10 = 2490
Step 250: 2490 + 10 = 2500
Step 251: 2500 + 10 = 2510
Step 252: 2510 + 10 = 2520
Step 253: 2520 + 10 = 2530
Step 254: 2530 + 10 = 2540
Step 255: 2540 + 10 = 2550
Step 256: 2550 + 10 = 2560
Step 257: 2560 + 10 = 2570
Step 258: 2570 + 10 = 2580
Step 259: 2580 + 10 = 2590
Step 260: 2590 + 10 = 2600
Step 261: 2600 + 10 = 2610
Step 262: 2610 + 10 = 2620
Step 263: 2620 + 10 = 2630
Step 264: 2630 + 10 = 2640
Step 265: 2640 + 10 = 2650
Step 266: 2650 + 10 = 2660
Step 267: 2660 + 10 = 2670
Step 268: 2670 + 10 = 2680
Step 269: 2680 + 10 = 2690
Step 270: 2690 + 10 = 2700
Step 271: 2700 + 10 = 2710
Step 272: 2710 + 10 = 2720
Step 273: 2720 + 10 = 2730
Step 274: 2730 + 10 = 2740
Step 275: 2740 + 10 = 2750
Step 276: 2750 + 10 = 2760
Step 277: 2760 + 10 = 2770
Step 278: 2770 + 10 = 2780
Step 279: 2780 + 10 = 2790
Step 280: 2790 + 10 = 2800
Step 281: 2800 + 10 = 2810
Step 282: 2810 + 10 = 2820
Step 283: 2820 + 10 = 2830
Step 284: 2830 + 10 = 2840
Step 285: 2840 + 10 = 2850
Step 286: 2850 + 10 = 2860
Step 287: 2860 + 10 = 2870
Step 288: 2870 + 10 = 2880
Step 289: 2880 + 10 = 2890
Step 290: 2890 + 10 = 2900
Step 291: 2900 + 10 = 2910
Step 292: 2910 + 10 = 2920
Step 293: 2920 + 10 = 2930
Step 294: 2930 + 10 = 2940
Step 295: 2940 + 10 = 2950
Step 296: 2950 + 10 = 2960
Step 297: 2960 + 10 = 2970
Step 298: 2970 + 10 = 2980
Step 299: 2980 + 10 = 2990
Step 300: 2990 + 10 = 3000
Step 301: 3000 + 10 = 3010
Step 302: 3010 + 10 = 3020
Step 303: 3020 + 10 = 3030
Step 304: 3030 + 10 = 3040
Step 305: 3040 + 10 = 3050
Step 306: 3050 + 10 = 3060
Step 307: 3060 + 10 = 3070
Step 308: 3070 + 10 = 3080
Step 309: 3080 + 10 = 3090
Step 310: 3090 + 10 = 3100
Step 311: 3100 + 10 = 3110
Step 312: 3110 + 10 = 3120
Step 313: 3120 + 10 = 3130
Step 314: 3130 + 10 = 3140
Step 315: 3140 + 10 = 3150
Step 316: 3150 + 10 = 3160
Step 317: 3160 + 10 = 3170
Step 318: 3170 + 10 = 3180
Step 319: 3180 + 10 = 3190
Step 320: 3190 + 10 = 3200
Step 321: 3200 + 10 = 3210
Step 322: 3210 + 10 = 3220
Step 323: 3220 + 10 = 3230
Step 324: 3230 + 10 = 3240
Step 325: 3240 + 10 = 3250
Step 326: 3250 + 10 = 3260
Step 327: 3260 + 10 = 3270
Step 328: 3270 + 10 = 3280
Step 329: 3280 + 10 = 3290
Step 330: 3290 + 10 = 3300
Step 331: 3300 + 10 = 3310
Step 332: 3310 + 10 = 3320
Step 333: 3320 + 10 = 3330
Step 334: 3330 + 10 = 3340
Step 335: 3340 + 10 = 3350
Step 336: 3350 + 10 = 3360
Step 337: 3360 + 10 = 3370
Step 338: 3370 + 10 = 3380
Step 339: 3380 + 10 = 3390
Step 340: 3390 + 10 = 3400
Step 341: 3400 + 10 = 3410
Step 342: 3410 + 10 = 3420
Step 343: 3420 + 10 = 3430
Step 344: 3430 + 10 = 3440
Step 345: 3440 + 10 = 3450
Step 346: 3450 + 10 = 3460
Step 347: 3460 + 10 = 3470
Step 348: 3470 + 10 = 3480
Step 349: 3480 + 10 = 3490
Step 350: 3490 + 10 = 3500
Step 351: 3500 + 10 = 3510
Step 352: 3510 + 10 = 3520
Step 353: 3520 + 10 = 3530
Step 354: 3530 + 10 = 3540
Step 355: 3540 + 10 = 3550
Step 356: 3550 + 10 = 3560
Step 357: 3560 + 10 = 3570
Step 358: 3570 + 10 = 3580
Step 359: 3580 + 10 = 3590
Step 360: 3590 + 10 = 3600
Step 361: 3600 + 10 = 3610
Step 362: 3610 + 10 = 3620
Step 363: 3620 + 10 = 3630
Step 364: 3630 + 10 = 3640
Step 365: 3640 + 10 = 3650
Step 366: 3650 + 10 = 3660
Step 367: 3660 + 10 = 3670
Step 368: 3670 + 10 = 3680
Step 369: 3680 + 10 = 3690
Step 370: 3690 + 10 = 3700
Step 371: 3700 + 10 = 3710
Step 372: 3710 + 10 = 3720
Step 373: 3720 + 10 = 3730
Step 374: 3730 + 10 = 3740
Step 375: 3740 + 10 = 3750
Step 376: 3750 + 10 = 3760
Step 377: 3760 + 10 = 3770
Step 378: 3770 + 10 = 3780
Step 379: 3780 + 10 = 3790
Step 380: 3790 + 10 = 3800
Step 381: 3800 + 10 = 3810
Step 382: 3810 + 10 = 3820
Step 383: 3820 + 10 = 3830
Step 384: 3830 + 10 = 3840
Step 385: 3840 + 10 = 3850
Step 386: 3850 + 10 = 3860
Step 387: 3860 + 10 = 3870
Step 388: 3870 + 10 = 3880
Step 389: 3880 + 10 = 3890
Step 390: 3890 + 10 = 3900
Step 391: 3900 + 10 = 3910
Step 392: 3910 + 10 = 3920
Step 393: 3920 + 10 = 3930
Step 394: 3930 + 10 = 3940
Step 395: 3940 + 10 = 3950
Step 396: 3950 + 10 = 3960
Step 397: 3960 + 10 = 3970
Step 398: 3970 + 10 = 3980
Step 399: 3980 + 10 = 3990

So, 10*399 = 3990