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: if a sum of money doubles itself in 20 years at simple interest, the ratepercent per annum is
Options: a. 12, b. 12.5, c. 13, d. 5, e. 14
[EX A]: d

[EX Q]: Problem: the perimeter of a triangle is 40 cm and the inradius of the triangle is 2.5 cm. what is the area of the triangle ?
Options: a. 87 cm 2, b. 28 cm 2, c. 98 cm 2, d. 26 cm 2, e. 50 cm 2
[EX A]: e

[EX Q]: Problem: solution a is 30 % salt and solution b is 80 % salt. if you have 30 ounces of solution a and 60 ounces of solution b, in what ratio could you mix solution a with solution b to produce 50 ounces of a 50 % salt solution ?
Options: a. 6: 4, b. 6: 14, c. 4: 4, d. 4: 6, e. 3: 2
[EX A]:
e