OK, let's work through this together. We're starting with 636 and we're going to multiply it by 25, which means we add 636 to itself 25 times.
Step 1: 0 + 636 = 636
Step 2: 636 + 636 = 1272
Step 3: 1272 + 636 = 1908
Step 4: 1908 + 636 = 2544
Step 5: 2544 + 636 = 3180
Step 6: 3180 + 636 = 3816
Step 7: 3816 + 636 = 4452
Step 8: 4452 + 636 = 5088
Step 9: 5088 + 636 = 5724
Step 10: 5724 + 636 = 6360
Step 11: 6360 + 636 = 6996
Step 12: 6996 + 636 = 7632
Step 13: 7632 + 636 = 8268
Step 14: 8268 + 636 = 8904
Step 15: 8904 + 636 = 9540
Step 16: 9540 + 636 = 10176
Step 17: 10176 + 636 = 10812
Step 18: 10812 + 636 = 11448
Step 19: 11448 + 636 = 12084
Step 20: 12084 + 636 = 12720
Step 21: 12720 + 636 = 13356
Step 22: 13356 + 636 = 13992
Step 23: 13992 + 636 = 14628
Step 24: 14628 + 636 = 15264
Step 25: 15264 + 636 = 15900

So, 636*25 = 15900