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:  A) 2  B) 4  C) 5  D) 67  E) 9
1/7 + 1/14 + 1/28 = 7/28 = 1/4 =&gt; 4 days   Answer: 4
<result>B</result>
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?