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.

[Q]: Problem: cost is expressed by the formula tb ^ 4. if b is doubled, the new cost w is what percent of the original cost ?
Options: a. w = 200, b. w = 600, c. w = 800, d. w = 1600, e. w = 50
[A]: d


[Q]: 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
[A]: c


[Q]: Problem: a, b, c and d enter into partnership. a subscribes 1 / 3 of the capital b 1 / 4, c 1 / 5 and d the rest. how much share did a get in a profit of rs. 2490 ?
Options: a. rs. 832, b. rs. 830, c. rs. 822, d. rs. 812, e. rs. 810
[A]:
b