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: a train 50 m long takes 6 sec to cross a man walking at 5 kmph in a direction opposite to that of the train. find the speed of the train ?
Options: a. 15 kmph, b. 25 kmph, c. 35 kmph, d. 45 kmph, e. 55 kmph
[EX A]: b

[EX Q]: Problem: if the simple interest on a certain sum of money for 6 years is one – fifth of the sum, then the rate of interest per annum is
Options: a. 3.3 %, b. 7.2 %, c. 6.1 %, d. 5.0 %, e. 3.1 %
[EX A]: a

[EX Q]: Problem: the ratio of the volumes of two cubes is 216: 343. what is the ratio of their total surface areas ?
Options: a. 81: 12, b. 31: 36, c. 81: 22, d. 8: 19, e. 1: 11
[EX A]:
b