Sophia has 2 tanks. Tank K and Tank L are of different capacities. If Tank K is filled by a tap at a rate of 4 litres per minute and Tank L is filled by a tap at a rate of 7 litres per minute, when Tank K is completely filled, 7 litres of water flowed out from Tank L. If Tank K is filled by a tap at a rate of 6 litres per minute and Tank L is filled by a tap at a rate of 5 litres per minute, when Tank K is completely filled, Tank L is only half-filled. What is the capacity of Tank L? Express your answer in litres.

	Scenario 1		Scenario 2
Tank type	Tank K	Tank L	Tank K	Tank L
Volume of the tank	4 u	7 u - 7	6 p	5 p x 2 = 10 p

Let 1 u be the time taken for Scenario 1.
Let 1 p be the time taken for Scenario 2.

4 u = 6 p --- (1)
7 u - 7 = 10 p
7 u = 10 p + 7 --- (2)

Make u the same.

(1) x 7
28 u = 42 p --- (3)

(2) x 4
28 u = 40 p + 28 --- (4)

(3) = (4)
42 p = 40 p + 28
42 p - 40 p = 28
2 p = 28
1 p = 28 ÷ 2 = 14

Volume of Tank L
= 10 p
= 10 x 14
= 140 ℓ

Answer(s): 140 ℓ