If a is congruent to 62 (mod 99) and b is congruent to 75 (mod 99), then for what integer n in the set {1000, 1001, 1002, 1003... 1098} is it true that a-b is congruent to n (mod 99)?
a−b≡n(mod99)62−75≡n(mod99)n≡−13(mod99)n≡86(mod99)n≡990+86(mod99)n≡1076(mod99)
So n is 1076
+586 
