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: a cement mixture is composed of 3 elements. by weight, 1 / 2 of the mixture is sand, 1 / 5 of the mixture is water, and the remaining 15 pounds of the mixture is gravel. what is the weight of the entire mixture in pounds ?
Options: a. 30, b. 40, c. 50, d. 60, e. 80
[A]: c

[Q]: Problem: micheal and adam can do together a piece of work in 20 days. after they have worked together for 15 days micheal stops and adam completes the remaining work in 10 days. in how many days micheal complete the work separately.
Options: a. 40 days, b. 100 days, c. 120 days, d. 110 days, e. 90 days
[A]: a

[Q]: Problem: two goods trains each 820 m long are running in opposite directions on parallel tracks. their speeds are 45 km / hr and 30 km / hr respectively. find the time taken by the slower train to pass the driver of the faster one ?
Options: a. 228, b. 27.32, c. 76.6, d. 78.72, e. 21
[A]:
d