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.

[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
[A]: a


[Q]: Problem: if a - b = 6 and a 2 + b 2 = 50, find the value of ab.
Options: a. a. 7, b. b. 12, c. c. 15, d. d. 18, e. e. 20
[A]: a


[Q]: Problem: a train speeds past a pole in 15 seconds and a platform 100 m long in 30 seconds. its length is ?
Options: a. 188 m, b. 876 m, c. 251 m, d. 200 m, e. 145 m
[A]:
d