X does a work in 15 days. Y does the same work in 45 days. In how many days they together will do the same work? Choose one.
A) 10  B) 12  C) 20  D) 11.25  E) 15

Rationale: "x ' s 1 day ' s work = 1 / 15 y ' s 1 day ' s work = 1 / 45 ( x + y ) ' s 1 day ' s work = ( 1 / 15 + 1 / 45 ) = 1 / 11.25 both together will finish the work in 11.25 days . correct option is d"
Answer: D