You are given a math word problem and you are supposed to make use of mathematical concepts like probability, combinatorics, linear equations, sequences/series, or simple operators like addition, subtraction, division, or multiplication to answer the given question. Choose the best option from the 5 given options and report that option.

Question: A man can do a piece of work in 5 days, but with the help of his son he can do it in 3 days. In what time can the son do it alone ?

 Option A: 13/2

 Option B: 19/2

 Option C: 21/2

 Option D: 15/2

 Option E: None
Option C

Question: Two pipes A and B can fill a cistern in 20 and 30 minutes respectively, and a third pipe C can empty it in 40 minutes. How long will it take to fill the cistern if all the three are opened at the same time?

 Option A: 17 1/5

 Option B: 17 1/8

 Option C: 17 1/7

 Option D: 17 1/1

 Option E: 17 1/2
Option C

Question: The Cottage Grove Loop Line travels the distance between the north side and the south side of the city in two hours, at a rate of speed of 100 km/h. How long is a round trip, measured in kilometers?

 Option A: 100 km

 Option B: 150 km

 Option C: 200 km

 Option D: 300 km

 Option E: 400 km

Option E