1a) Since 25 divides evenly into 100, and 100 divides evenly into 7100, there must be a zero remainder

1b) 2603*3201 = (2600+3)*(3200+1) = (2600*3200 + 2600 + 3*3200 + 3) = 3 (mod 25)

1c) My Maple says:

29 * 7100 + 54 * 2603 * 3201 mod 25 = 12

So you've typed it in wrong.

In 2) I think ABE=0, because you need B=D and E=A to satisfy the conditions of the problem.