A man can do a piece of work in 6 days, but with the help of his son, he can do it in 3 days. In what time can the son do it alone? Choose one:
A) 5  B) 7  C) 6  D) 8  E) 9

Rationale: "son ' s 1 day ' s work = ( 1 / 3 ) - ( 1 / 6 ) = 1 / 6 the son alone can do the work in 6 days answer is c"
Answer: C