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:  A) 2 days  B) 6 days  C) 12 days  D) 20 days  E) none of these

Rationale: "explanation : in this type of question , where we have one person work and together work done . then we can easily get the other person work just by subtracting them . as son ' s one day work = ( 1 / 4 − 1 / 5 ) = ( 5 − 4 ) / 20 = 1 / 20 so son will do whole work in 20 days answer : d"
Answer: D