Ella has 2 tanks. Tank L and Tank M are of different capacities. If Tank L is filled by a tap at a rate of 3 litres per minute and Tank M is filled by a tap at a rate of 7 litres per minute, when Tank L is completely filled, 5 litres of water flowed out from Tank M. If Tank L is filled by a tap at a rate of 6 litres per minute and Tank M is filled by a tap at a rate of 2 litres per minute, when Tank L is completely filled, Tank M is only one-fifth-filled. What is the capacity of Tank M? Express your answer in litres.

	Scenario 1		Scenario 2
Tank type	Tank L	Tank M	Tank L	Tank M
Volume of the tank	3 u	7 u - 5	6 p	2 p x 5 = 10 p

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

3 u = 6 p --- (1)
7 u - 5 = 10 p
7 u = 10 p + 5 --- (2)

Make u the same.

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

(2) x 3
21 u = 30 p + 15 --- (4)

(3) = (4)
42 p = 30 p + 15
42 p - 30 p = 15
12 p = 15
1 p = 15 ÷ 12 = 1.25

Volume of Tank M
= 10 p
= 10 x 1.25
= 12.5 ℓ

Answer(s): 12.5 ℓ