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 sum of money at simple interest amounts to $ 645 in 3 years and to $ 690 in 4 years. the sum is :
Options: a. $ 153, b. $ 510, c. $ 598, d. $ 549, e. $ 675
[EX A]: b

[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: the h. c. f. of two numbers is 25 and the other two factors of their l. c. m. are 13 and 14. the larger of the two numbers is :
Options: a. 276, b. 299, c. 322, d. 345, e. 350
[EX A]:
e