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: what is the normal price of an article sold at $ 72 after two successive discounts of 10 % and 20 % ?
Options: a. $ 92, b. $ 96, c. $ 100, d. $ 104, e. $ 108
[EX A]: c

[EX Q]: Problem: for any positive integer n, the sum of the first n positive integers equals n ( n + 1 ) / 2. what is the sum of all the integers between 100 and 200 ?
Options: a. 4545, b. 5050, c. 6525, d. 4352, e. 4652
[EX A]: b

[EX Q]: Problem: a searchlight on top of the watch - tower makes 3 revolutions per minute. what is the probability that a man appearing near the tower will stay in the dark for at least 15 seconds ?
Options: a. 1 / 4, b. 1 / 3, c. 1 / 2, d. 2 / 3, e. 3 / 4
[EX A]:
a