A and B together can complete a piece of work in 10days. If A alone can complete the same work in 15days, in how many days, in how many days can B alone complete that work?  Choose one.
A) 10  B) 30  C) 25  D) 15  E) 45

Rationale: ( a + b ) ' s 1 day ' s work = 1 / 10 a ' s 1 day ' s work = 1 / 15 b ' s 1 days ' s work = ( 1 / 10 ) - ( 1 / 15 ) = 1 / 30 hence , b alone can complete the work in 30 days . answer is b
Answer: B