A,B,C can complete a piece of work in 21,6,12 days. Working together, they complete the same work in how many days? Choose the correct choice from the following:  A) 2  B) 4 / 5  C) 7 / 9  D) 10  E) 84 / 25

Rationale: "a + b + c 1 day work = 1 / 21 + 1 / 6 + 1 / 12 = 25 / 84 a , b , c together will complete the job in 84 / 25 days answer is e"
Answer: E