The task is to generate the correct answer for a given math problem.

Problem: excluding stoppages , the average speed of a bus is 60 km / hr and including stoppages , the average speed of the bus is 15 km / hr . for how many minutes does the bus stop per hour ?
4

Problem: a and b can do a work in 5 days and 10 days respectively . a starts the work and b joins him after 2 days . in how many days can they complete the remaining work ?
2

Problem: what least number should be subtracted from 13600 such that the remainder is divisible by 87 ?

28