For the four-digit number ABCD (a string, not a product), where A,B,C, and D all represent distinct digits, find the value of B, given ABCD rounded to the nearest hundred is CD00.

4950, 5960, 6970 and 7980 all work. So B must be 9. But actually, all we need to know is that A changes when rounding to the nearest hundred, so the number must be rounded up, and the only time A changes when B changes is when B is 9.