Solutions
Given:
P can complete a task in 20 days,
Q in 30 days,
R in 60 days,
P is helped by Q and R every third day.
Concept:
Work done is reciprocal of the time taken. Use the concept of man-days.
Solution:
Total work = LCM(20, 30, 60) = 60 units
Efficiency of P = 60/20 = 3
Efficiency of Q = 60/30 = 2
Efficiency of R = 60/60 = 1
Work done by P in two days = 2 days × 3 = 6 units.
On third day, all three work together, their combined work = 3 + 2 + 1 = 6 units.
Therefore, in every cycle of three days, work done = 6 + 6 = 12 units.
To finish 60 unit of work, days required = 3 × 5 = 15 days.
Hence, it will take 15 days to finish the work.