Let's get this math done. We have 845 and we're going to multiply it by 20. This is the same as taking 845 and adding it to itself 20 times.
Step 1: 0 + 845 = 845
Step 2: 845 + 845 = 1690
Step 3: 1690 + 845 = 2535
Step 4: 2535 + 845 = 3380
Step 5: 3380 + 845 = 4225
Step 6: 4225 + 845 = 5070
Step 7: 5070 + 845 = 5915
Step 8: 5915 + 845 = 6760
Step 9: 6760 + 845 = 7605
Step 10: 7605 + 845 = 8450
Step 11: 8450 + 845 = 9295
Step 12: 9295 + 845 = 10140
Step 13: 10140 + 845 = 10985
Step 14: 10985 + 845 = 11830
Step 15: 11830 + 845 = 12675
Step 16: 12675 + 845 = 13520
Step 17: 13520 + 845 = 14365
Step 18: 14365 + 845 = 15210
Step 19: 15210 + 845 = 16055
Step 20: 16055 + 845 = 16900

So, 845*20 = 16900