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?  Pick.
A) 6 days
B) 4 days
C) 8 days
D) 5 days
E) 1 days
1/7 + 1/14 + 1/28 = 7/28 = 1/4   =&gt; 4 days   Answer: 4 days
<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?