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.

Input: Consider Input: Problem: a sum of money at simple interest amounts to rs. 720 after 2 years and to rs. 1020 after a further period of 5 years. the sum is :
Options: a. 400, b. 500, c. 600, d. 700, e. 800

Output: c


Input: Consider Input: Problem: if n = 2 ^ 0.15 and n ^ b = 1, b must equal
Options: a. 3 / 80, b. 20 / 3, c. 4, d. 5 / 3, e. 80 / 3

Output: b


Input: Consider Input: Problem: a certain fraction is equivalent to 3 / 5. if the numerator of the fraction is increased by 4 and the denominator is doubled, the new fraction is equivalent to 1 / 3. what is the sum of the numerator and denominator of the original fraction ?
Options: a. 49, b. 35, c. 28, d. 56, e. 21


Output: d