OK, let's work through this together. We're starting with 90 and we're going to multiply it by 40, which means we add 90 to itself 40 times.
Step 1: 0 + 90 = 90
Step 2: 90 + 90 = 180
Step 3: 180 + 90 = 270
Step 4: 270 + 90 = 360
Step 5: 360 + 90 = 450
Step 6: 450 + 90 = 540
Step 7: 540 + 90 = 630
Step 8: 630 + 90 = 720
Step 9: 720 + 90 = 810
Step 10: 810 + 90 = 900
Step 11: 900 + 90 = 990
Step 12: 990 + 90 = 1080
Step 13: 1080 + 90 = 1170
Step 14: 1170 + 90 = 1260
Step 15: 1260 + 90 = 1350
Step 16: 1350 + 90 = 1440
Step 17: 1440 + 90 = 1530
Step 18: 1530 + 90 = 1620
Step 19: 1620 + 90 = 1710
Step 20: 1710 + 90 = 1800
Step 21: 1800 + 90 = 1890
Step 22: 1890 + 90 = 1980
Step 23: 1980 + 90 = 2070
Step 24: 2070 + 90 = 2160
Step 25: 2160 + 90 = 2250
Step 26: 2250 + 90 = 2340
Step 27: 2340 + 90 = 2430
Step 28: 2430 + 90 = 2520
Step 29: 2520 + 90 = 2610
Step 30: 2610 + 90 = 2700
Step 31: 2700 + 90 = 2790
Step 32: 2790 + 90 = 2880
Step 33: 2880 + 90 = 2970
Step 34: 2970 + 90 = 3060
Step 35: 3060 + 90 = 3150
Step 36: 3150 + 90 = 3240
Step 37: 3240 + 90 = 3330
Step 38: 3330 + 90 = 3420
Step 39: 3420 + 90 = 3510
Step 40: 3510 + 90 = 3600

So, 90*40 = 3600