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: excluding stoppages, the average speed of a bus is 60 km / hr and including stoppages, the average speed of the bus is 40 km / hr. for how many minutes does the bus stop per hour ?
Options: a. 18 min, b. 17 min, c. 19 min, d. 20 min, e. 21 min
[EX A]: d

[EX Q]: Problem: the present worth of rs. 2310 due years hence, the rate of interest being 15 % per annum, is :
Options: a. rs. 1440, b. rs. 1750, c. rs. 1680, d. rs. 1840, e. none of these
[EX A]: c

[EX Q]: Problem: if it would take one machine 20 minutes to fill a large production order and another machine 30 minutes to fill the same order, how many minutes would it take both machines working together, at their respective rates, to fill the order ?
Options: a. 10 min, b. 12 min, c. 15 min, d. 20 min, e. 18 min
[EX A]:
b