A can do a piece of work in 7 hours, B and C can do it in 21/4 hours, and C and A can do it in 7/2 hours. How long will B alone take to do it?
Show answer & explanation
Correct answer: B
Explanation
The correct answer is Option 2 i.e. 21 hours.
A can do a piece of work in 7 hours, B and C can do it in 21/4 hours, and C and A can do it in 7/2 hours
L.C.M(7, 21/4, and 7/2) = 84
The efficiency of A = 84/7 = 12 units
The efficiency of B and C = 84/(21/4) = (84 × 4)/21 = 16 units
The efficiency of C and A = 84/7/2 = (84 × 2)/7 = 24 units
So,
The efficiency of C = 24 - 12 = 12 units
The efficiency of B = 16 - 12 = 4 units
Thus,
Time taken by B = 84/4 = 21 hours