A works twice as fast as B. If B can complete a work in 12 days independently, the number of days in which A and B can together finish the work in :  Choose the correct choice from the following choices:
A) 4 days	B) 6 days	C) 8 days	D) 18 days	E) 28 days

Rationale: "explanation : ratio of rates of working of a and b = 2 : 1 . so , ratio of times taken = 1 : 2 . b ' s 1 day ' s work = 1 / 12 a ' s 1 day ' s work = 1 / 6 ; ( 2 times of b ' s work ) ( a + b ) ' s 1 day ' s work = ( 1 / 6 + 1 / 12 ) = 3 / 12 = 1 / 4 so , a and b together can finish the work in 4 days . answer is a"
Answer: A