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.

Problem: the overall age of x and y is 19 year greater than the overall age of y and z. z is how many decades younger that x ?
Options: a. 11, b. 15, c. 12, d. 19, e. 20
d

Problem: if a - b = 6 and a 2 + b 2 = 50, find the value of ab.
Options: a. a. 7, b. b. 12, c. c. 15, d. d. 18, e. e. 20
a

Problem: thirty percent of the members of a swim club have passed the lifesaving test. among the members who have not passed the test, 12 have taken the preparatory course and 30 have not taken the course. how many members are there in the swim club ?
Options: a. 60, b. 80, c. 100, d. 120, e. 140

a