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: 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
d

Problem: the difference between the compound interest compounded annually and simple interest for 2 years at 20 % per annum is rs. 216. find the principal ?
Options: a. 2277, b. 2667, c. 5400, d. 2766, e. 1811
c

Problem: john found that the average of 15 no. ' s is 40. if 10 is added to each number then the mean of number is ?
Options: a. 50, b. 55, c. 65, d. 72, e. 78

a