A man can do a piece of work in 5 days, but with the help of his son, he can do it in 4 days. In what time can the son do it alone?  Choose the correct choice from the following options.
A) 15	B) 17	C) 20	D) 8	E) 9

Rationale: "son ' s 1 day ' s work = ( 1 / 4 ) - ( 1 / 5 ) = 1 / 20 the son alone can do the work in 20 days answer is c"
Answer: C