A, B and C can do a piece of work in 7 days, 21 days and 28 days respectively. How long will they taken, if all the three work together?  Answers:
A) 2  B) 4 8/19  C) 5  D) 67  E) 9
1/7 + 1/21 + 1/28 = 19/84 =&gt; 84/19 =&gt; 4 8/19 days   Answer: 4 8/19
<result>B</result>
A, B and C can do a piece of work in 7 days, 21 days and 28 days respectively. How long will they taken, if all the three work together?