Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

[EX Q]: Problem: in a group of 100 people, 90 have an age of more 30 years, and the others have an age of less than 20 years. if a person is selected at random from this group, what is the probability the person ' s age is less than 20 ?
Options: a. 0.1, b. 0.55, c. 0.65, d. 0.75, e. 0.85
[EX A]: a

[EX Q]: Problem: if john makes a contribution to a charity fund at school, the average contribution size will increase by 50 % reaching $ 75 per person. if there were 1 other contributions made before john ' s, what is the size of his donation ?
Options: a. $ 100, b. $ 150, c. $ 200, d. $ 250, e. $ 450
[EX A]: a

[EX Q]: Problem: john purchased some shirts and trousers for $ 700. he paid $ 400 less for the shirts than he did for the trousers. if he bought 5 shirts and the cost of a shirt is $ 20 less than that of a trouser, how many trousers did he buy ?
Options: a. 4, b. 5, c. 6, d. 7, e. 11
[EX A]:
e