Tony alone can paint a wall in 7 days and his friend Roy alone can paint the same wall in 9 days. In how many days they can paint the wall working together? Round off the answer to the nearest integer.	Pick.
A) 2	B) 4	C) 6	D) 7	E) 9

Rationale: sol . use formula ( xy / x + y ) so nearest value for 3.93 = 4 answer : option b
Answer: B