1 Answers
Option 5 : 30 hours
Given:
Pipe A can fill the tank in 20 hours
Pipe B can fill the tank in 40 hours
Pipe C can empty the tank in 60 hours
A works with 1/2 of its efficiency
B works with 1/3 of its efficiency
Concept used:
Work = Time taken × Efficiency
Total efficiency = Efficiency of Filling pipes – Efficiency of emptying pipes
Calculation:
Let the total work be x
Efficiency of pipe A = x/20
Efficiency of pipe B = x/40
Efficiency of pipe C = x/60
Now, after dust enters Pipes A and B
Efficiency of pipe A = x/20 × (1/2)
⇒ Efficiency of Pipe A = x/40
Efficiency of pipe B = x/40 × (1/3)
⇒ Efficiency of pipe B = x/120
Total efficiency = Efficiency of Filling pipes – Efficiency of emptying pipes
Now, Total efficiency of A + B + C = x/40 + x/120 – x/60
⇒ Total efficiency of A + B + C = 2x/120 = x/60
Part of tank which is already filled = x/2
Tank to be filled = x – x/2 = x/2
Time taken to fill the remaining part of tank = x/2 ÷ (x/60)
⇒ 30 hours
∴ Time taken to fill the whole tank if it is already half filled is 30 hours
Alternate solution:
A can fill the tank in 20 hours
B can fill the tank in 40 hours
C can empty the tank in 60 hours
L.C.M of 20, 40 and 60 = 120
Efficiency of A = 6
Efficiency of B = 3
Efficiency of C = 2
Now, after dust enters Pipes A and B
Efficiency of A = 6 × 1/2 = 3
Efficiency of B = 3 × 1/3 = 1
Efficiency of C = 2
Total efficiency = Efficiency of Filling pipes – Efficiency of emptying pipes
Total Efficiency of A + B + C = 3 + 1 – 4
⇒ Total Efficiency of A + B + C = 2
Part of tank which is already filled = 120/2 = 60
⇒ Part of tank which is to be filled = 120 – 60 = 60
Time taken to fill the remaining tank = 60/2
⇒ 30 hours
∴ Time taken to fill the whole tank if it is already half filled is 30 hours