Quantity A is the overlap of people who bought stamps and also mailed letters. Since the number of people who mailed letters was 7, the maximum possible overlap is 7 (i.e., if all 7 people who bought letters also bought stamps). It could also be as few as 0 (i.e., if there was no one who bought letters and also bought stamps).

Quantity B is given as 10.

Since the max possible value for Quantity A is 7, it will always be less than Quantity B.