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 mean proportional between 4 and 16 is ?
Options: a. 12, b. 13, c. 14, d. 15, e. 16
e

Problem: after giving a discount of rs. 45 the shopkeeper gets a loss of 10 %, if the cost price is rs. 180. find the markup % ?
Options: a. 0.21, b. 0.2, c. 0.17, d. 0.21, e. 0.15
e

Problem: if 3 / w + 3 / x = 3 / y and wx = y, then the average ( arithmetic mean ) of w and x is
Options: a. 1 / 2, b. 1, c. 2, d. 4, e. 8

a