If a can do a work in 10 days and a and b can do a same piece of job in 5 days. IN how days can do a work b alone?	Choose the most appropriate option
A) 10 B) 20 C) 5 D) 15 E) 9

Rationale: a ' s one day work = 1 / 10 a + b = 1 / 5 ( one day work ) b = ( 1 / 5 ) - a = ( 1 / 5 ) - ( 1 / 10 ) = 1 / 10 ( one day work ) b can done a work in 10 days . answer option a
Answer: A