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: The ratio of a to b to c is 2 to 3 to 4, and a, b, c are positive integers. If the average (arithmetic mean) of the three numbers is 36, what is the value of a?

 Option A: 21

 Option B: 24

 Option C: 25

 Option D: 27

 Option E: 30
Option B

Question: How many integers from 100 to 800, inclusive, remains the value unchanged when the digits were reversed?

 Option A:  50

 Option B:  60

 Option C:  80

 Option D: 70

 Option E:  90

Option D