Let's roll up our sleeves and solve this. We have 836 and we're going to multiply it by 25, essentially adding 836 to itself 25 times.
Step 1: 0 + 836 = 836
Step 2: 836 + 836 = 1672
Step 3: 1672 + 836 = 2508
Step 4: 2508 + 836 = 3344
Step 5: 3344 + 836 = 4180
Step 6: 4180 + 836 = 5016
Step 7: 5016 + 836 = 5852
Step 8: 5852 + 836 = 6688
Step 9: 6688 + 836 = 7524
Step 10: 7524 + 836 = 8360
Step 11: 8360 + 836 = 9196
Step 12: 9196 + 836 = 10032
Step 13: 10032 + 836 = 10868
Step 14: 10868 + 836 = 11704
Step 15: 11704 + 836 = 12540
Step 16: 12540 + 836 = 13376
Step 17: 13376 + 836 = 14212
Step 18: 14212 + 836 = 15048
Step 19: 15048 + 836 = 15884
Step 20: 15884 + 836 = 16720
Step 21: 16720 + 836 = 17556
Step 22: 17556 + 836 = 18392
Step 23: 18392 + 836 = 19228
Step 24: 19228 + 836 = 20064
Step 25: 20064 + 836 = 20900

So, 836*25 = 20900