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.

Problem: the total age of a and b is 15 years more than the total age of b and c. c is how many year younger than
Options: a. 14 years, b. 12 years, c. 56 years, d. 66 years, e. 15 years
e

Problem: what is the greatest positive integer n such that 3 ^ n is a factor of 36 ^ 100 ?
Options: a. 100, b. 200, c. 300, d. 600, e. 900
b

Problem: a man purchased earphone on friday. but he forgot how much it cost. the data he have are mean expenditure of all 7 days is 500. expenditure on mon, tue, wed, thurs, sat, sun are 450600, 400500, 550300. on friday he purchased 3 things one earphone, one pen and one notebook. pen cost 30 and notebook cost 50. then what is cost of earphone ?
Options: a. 620, b. 530, c. 450, d. 470, e. 500
a