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 - 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
[EX A]: a

[EX Q]: Problem: a man spends 2 / 5 of his salary on house rent, 3 / 10 of his salary on food and 1 / 8 of his salary on convenience he has rs. 1400 left with him, find his expenditure on food and conveyance.
Options: a. 1000, b. 3000, c. 4000, d. 2000, e. 5000
[EX A]: a

[EX Q]: Problem: a fair coin is tossed 5 times. what is the probability that it lands heads up at least twice ?
Options: a. 1 / 16, b. 5 / 16, c. 2 / 5, d. 13 / 16, e. 27 / 32
[EX A]:
d