A, B and C can do a piece of work in 7 days, 14 days and 28 days respectively. How long will they taken, if all the three work together? Choose the correct choice from the following answers.
A) 4
B) 3
C) 5
D) 2
E) 1

Rationale: "1 / 7 + 1 / 14 + 1 / 28 = 7 / 28 = 1 / 4 = > 4 days answer a"
Answer: A