Shortcut Trick
Given P + R = 2Q. Adding Q to both sides gives P + Q + R = 3Q.
The combined efficiency of the group is 3 times the efficiency of Q alone.
Since time is inversely proportional to efficiency, Q will take 3 times the time taken by the group.
Time for Q alone = 4 days × 3 = 12 days.
∴ The correct answer is 12 days.
Alternate Method
Given:
(Q + R) = 6 × P
(P + R) = 2 × Q
Time (P + Q + R) = 4 days
Formula Used:
Total Work = Combined Efficiency × Time Taken
Calculations:
⇒ Efficiency (Q + R) : P = 6 : 1 (Total parts = 7)
⇒ Efficiency (P + R) : Q = 2 : 1 (Total parts = 3)
⇒ To equalize total efficiency, use LCM of 7 and 3 = 21.
⇒ Total Efficiency (P + Q + R) = 21 units/day.
⇒ Efficiency of P = 21 × (1 ÷ 7) = 3 units/day.
⇒ Efficiency of Q = 21 × (1 ÷ 3) = 7 units/day.
⇒ Efficiency of R = 21 − (3 + 7) = 11 units/day.
⇒ Total Work = 21 units/day × 4 days = 84 units.
⇒ Time for Q alone = Total Work ÷ Efficiency of Q
⇒ Time = 84 ÷ 7 = 12 days.
∴ The correct answer is 12 days.
Additional Information
Efficiency and Time Relationship
Efficiency is inversely proportional to the time taken to complete a task. If efficiency is doubled, time is halved (Time = Work ÷ Efficiency).
Combined Rate Formula
When multiple people work together, their individual efficiencies are added to get the total efficiency: Etotal = E1 + E2 + ... + En.
Ratio Normalization
To solve efficiency ratio problems involving multiple groupings, normalize the total parts of the ratios by using the Least Common Multiple (LCM).