X does a work in 20 days. Y does the same work in 30 days. In how many days they together will do the same work?	Choices:
A) 10 B) 12 C) 20 D) 30 E) 15

Rationale: "x ' s 1 day ' s work = 1 / 20 y ' s 1 day ' s work = 1 / 30 ( x + y ) ' s 1 day ' s work = ( 1 / 20 + 1 / 30 ) = 1 / 12 both together will finish the work in 12 days . correct option is b"
Answer: B