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 certain sum is invested at simple interest at 18 % p. a. for two years instead of investing at 12 % p. a. for the same time period. therefore the interest received is more by rs. 300. find the sum ?
Options: a. 7000, b. 2500, c. 2778, d. 2800, e. 2791
[EX A]: b

[EX Q]: Problem: if p ( a. = 5 / 7 and p ( b. = 2 / 5, find p ( a n b. if a and b are independent events.
Options: a. 7 / 25, b. 3 / 25, c. 8 / 25, d. 2 / 7, e. 3 / 17
[EX A]: d

[EX Q]: Problem: at a certain restaurant, the average ( arithmetic mean ) number of customers served for the past x days was 65. if the restaurant serves 120 customers today, raising the average to 90 customers per day, what is the value of x ?
Options: a. 1, b. 5, c. 9, d. 15, e. 30
[EX A]:
a