OK, let's work through this together. We're starting with 120 and we're going to multiply it by 21, which means we add 120 to itself 21 times.
Step 1: 0 + 120 = 120
Step 2: 120 + 120 = 240
Step 3: 240 + 120 = 360
Step 4: 360 + 120 = 480
Step 5: 480 + 120 = 600
Step 6: 600 + 120 = 720
Step 7: 720 + 120 = 840
Step 8: 840 + 120 = 960
Step 9: 960 + 120 = 1080
Step 10: 1080 + 120 = 1200
Step 11: 1200 + 120 = 1320
Step 12: 1320 + 120 = 1440
Step 13: 1440 + 120 = 1560
Step 14: 1560 + 120 = 1680
Step 15: 1680 + 120 = 1800
Step 16: 1800 + 120 = 1920
Step 17: 1920 + 120 = 2040
Step 18: 2040 + 120 = 2160
Step 19: 2160 + 120 = 2280
Step 20: 2280 + 120 = 2400
Step 21: 2400 + 120 = 2520

So, 120*21 = 2520