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.

Example Input: Problem: john can complete a given task in 18 days. jane will take only 12 days to complete the same task. john and jane set out to complete the task by beginning to work together. however, jane was indisposed 6 days before the work got over. in how many days did the work get over from the time john and jane started to work on it together ?
Options: a. 11, b. 34 / 5, c. 44 / 5, d. 54 / 5, e. 12
Example Output: d

Example Input: Problem: a solution of 70 litres contains oil and water in the ratio 9: x. if 6 litres of water is added to the solution, the ratio becomes 6: 4, find the value of x ?
Options: a. 6, b. 8.5, c. 9, d. 6.5, e. 5
Example Output: e

Example Input: Problem: if the operation @ is defined for all a and b by the equation a @ b = ( a - b. / 5, then ( 3 @ - 2 ) @ 6 = ?
Options: a. 0, b. 1, c. 2, d. - 2, e. - 1
Example Output:
e