Given:
Time taken by pipe A to fill a half filled tank = 6 hours
Time taken by pipe B to fill a quarter filled tank = 6 hours
Concept used:
If a pipe fills a tank in t hours, then quantity of water filled by the pipe in 1 hour = 1 / t
Solution:
Time taken by pipe A to fill half the tank alone = 6 hours.
Thus, time taken by pipe A to fill an empty tank alone = 6 × 2 = 12 hours.
Time taken by pipe B to fill a quarter filled tank = 6 hours
Quantity of tank filled by B in 6 hours
= [1 – (1 / 4)]th of tank
= (3 / 4)th of tank
Time taken by pipe B to fill an empty tank alone
= (4 / 3) × 6
= 4 × 2
= 8 hours.
Let the quantity of the tank be 24 litres (As LCM (12, 8) = 24
Amount left to be filled when three-eighth of the tank is filled already
= (1 – 3 / 8)th of 24
= (5 / 8) × 24
= 15 litres.
Quantity of tank filled by pipe A alone in 1 hour
= (1 / 12) of capacity of tank
Quantity of tank filled by pipe B alone in 1 hour
= (1 / 8) of capacity of tank
Quantity of tank filled by pipe A and B together in 1 hour
= 1 / 12 + 1 / 8
= (2 + 3) / 24
= (5 / 24) of capacity of tank
= (5 / 24) × 24
= 5 litres.
Time taken for A and B to fill the remaining 15 litres = 15 / 5 = 3 hours.
∴ Working together it will take 3 hours to fill up the tank when it is already three-eighth filled.
Alternate Method
Given:
Time taken by pipe A to fill a half filled tank = 6 hours
Time taken by pipe B to fill a quarter filled tank = 6 hours
Concept used:
In LCM method we find the LCM of appropriate entities then use their efficiencies to get the required answer.
Solution:
Time taken by pipe A to fill half the tank alone = 6 hours.
Thus, time taken by pipe A to fill an empty tank alone = 6 × 2 = 12 hours.
Time taken by pipe B to fill a quarter filled tank = 6 hours
Quantity of tank filled by B in 6 hours
= [1 – (1 / 4)]th of tank
= (3 / 4)th of tank
Time taken by pipe B to fill an empty tank alone
= (4 / 3) × 6
= 4 × 2
= 8 hours.
The numbers above the line indicate the efficiencies and the numbers above the names represent the time taken to fill or empty the tank. The number on the right indicate the total capacity of the tank.
Let the total capacity of tank = 24 (LCM of 12 and 8)
Efficiency of pipe A = 24 / 12 = + 2.
Efficiency of pipe B = 24 / 8 = + 3.
Sum of efficiencies of A and B = 2 + 3 = 5.
Amount left to be filled when three-eighth of the tank is filled already
= (1 – 3 / 8)th of 24
= (5 / 8) × 24
= 15 litres.
Time taken by A and B together to fill 15 litres = 15 / 5 = 3 hours.
∴ Working together it will take 3 hours to fill up the tank when it is already three-eighth filled.