A works twice as fast as B.If B can complete a work in 18 days independently,the number of days in which A and B can together finish the work is:  Choose the correct choice from the following options:
A) 4 days	B) 6 days	C) 8 days	D) 10 days	E) 12 days

Rationale: "explanation : ratio of rates of working of a and b = 2 : 1 . so , ratio of times taken = 1 : 2 a ' s 1 day ' s work = 1 / 9 b ' s 1 day ' s work = 1 / 18 ( a + b ) ' s 1 day ' s work = ( 1 / 9 + 1 / 18 ) = 3 / 18 = 1 / 6 so , a and b together can finish the work in 6 days answer is b"
Answer: B