A can do a piece of work in 12 days and B can do a piece of work in 18 days how many days will the work be completed if both of them work together? Select.
A) 15 B) 7.2 C) 7.7 D) 7.4 E) 8

Rationale: "a ' s 1 day work = 1 / 12 , b ' s 1 day work = 1 / 18 ; ( a + b ) 1 day work = 1 / 12 + 1 / 18 = 5 / 36 ; they can finish the work in = 36 / 5 = 7.2 answer : b"
Answer: B