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: a trader mixes 80 kg of tea at 15 per kg with 20 kg of tea at cost price of 20 per kg. in order to earn a profit of 25 %, what should be the sale price of the mixed tea ?
Options: a. 23.75, b. 22, c. 20, d. 19.20, e. none of these
[EX A]: c

[EX Q]: Problem: how many 4 - digit numbers that do not contain the digits 2, 3, 4, 5, 6, or 7 are there ?
Options: a. 168, b. 176, c. 184, d. 192, e. 200
[EX A]: d

[EX Q]: Problem: what is 2 2 / 3 - 1 1 / 4 divided by 1 / 2 - 1 / 4 ?
Options: a. 17 / 36, b. 36 / 17, c. 17 / 6, d. 17 / 3, e. 51 / 4
[EX A]:
d