A man can do a piece of work in 7 days, but with the help of his son, he can do it in 3 days. In what time can the son do it alone? Choices:
A) 5	B) 7	C) 15 / 2	D) 21 / 4	E) 18 / 5

Rationale: "son ' s 1 day ' s work = ( 1 / 3 ) - ( 1 / 7 ) = 4 / 21 the son alone can do the work in 21 / 4 days answer is d"
Answer: D