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: dhoni bought 5 toys with the average cost of $ 10. if david also buys the sixth toy with the price of $ 16, what is the average ( arithmetic mean ) price of those 6 toys ?
Options: a. $ 10, b. $ 7, c. $ 8.5, d. $ 9, e. $ 11
e

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: if x * y = xy – 2 ( x + y ) for all integers x and y, then 1 * ( – 3 ) =
Options: a. – 16, b. – 11, c. – 4, d. 4, e. 1

e