P and Q together can complete a work in 21 days. Q alone can complete it in 84 days. If efficiency of Q becomes double, then in how many days approximately will P and Q together complete the work?
Show answer & explanation
Correct answer: D
Explanation
The correct answer is Option 4 i.e. 17 days
Calculations:
Number of days taken by P + Q = 21
Number of days taken by Q = 84
Total work done (LCM of 21 and 84) = 84 units
Efficiency of P + Q = 84/21 = 4
Efficiency of Q = 84/84 = 1
Efficiency of P = Efficiency of (P + Q) - Efficiency of Q
⇒ 4 - 1 = 3
According to the question;
Efficiency of Q is doubled = 2
Total new efficiency = 3 + 2 = 5
Required time taken = 84/5 = 16.8
⇒ 17 days (Approximately)