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: at what rate percent per annum will a sum of money double in 6 years.
Options: a. 12.5 %, b. 13.5 %, c. 11.5 %, d. 14.5 %, e. 16.6 %
e

Problem: the average age of 17 persons in a office is 15 years. out of these, the average age of 5 of them is 14 years and that of the other 9 persons is 16 years. the age of the 15 th person is ?
Options: a. 39, b. 41, c. 45, d. 42, e. 50
b

Problem: the average age of students of a class is 15.8 years. the average age of boys in the class is 16.5 years and that of the girls is 15.4 years. the ration of the number of boys to the number of girls in the class is :
Options: a. 2: 5, b. 2: 3, c. 2: 4, d. 2: 1, e. 4: 7
e