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: If the charge of staying in a student youth hostel $18.00/day for the first week, and $15.00/day for each additional week, How much does it cost to stay for 23 days?

 Option A: $336

 Option B: $289

 Option C: $282

 Option D: $274

 Option E: $286
Option A

Question: A boy wants to go abroad by boat and return by flight. He has a choice of 6 different boat to go and 5 flight to return. In how many ways, can the boy perform his journey?

 Option A: 20

 Option B: 25

 Option C: 30

 Option D: 40

 Option E: 50

Option C