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

	Scenario 1		Scenario 2
Tank type	Tank W	Tank X	Tank W	Tank X
Volume of the tank	4 u	5 u - 5	4 p	2 p x 2 = 4 p

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

4 u = 4 p --- (1)
5 u - 5 = 4 p
5 u = 4 p + 5 --- (2)

Make u the same.

(1) x 5
20 u = 20 p --- (3)

(2) x 4
20 u = 16 p + 20 --- (4)

(3) = (4)
20 p = 16 p + 20
20 p - 16 p = 20
4 p = 20
1 p = 20 ÷ 4 = 5

Volume of Tank X
= 4 p
= 4 x 5
= 20 ℓ

Answer(s): 20 ℓ