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

	Scenario 1		Scenario 2
Tank type	Tank P	Tank Q	Tank P	Tank Q
Volume of the tank	4 u	5 u - 7	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 - 7 = 4 p
5 u = 4 p + 7 --- (2)

Make u the same.

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

(2) x 4
20 u = 16 p + 28 --- (4)

(3) = (4)
20 p = 16 p + 28
20 p - 16 p = 28
4 p = 28
1 p = 28 ÷ 4 = 7

Volume of Tank Q
= 4 p
= 4 x 7
= 28 ℓ

Answer(s): 28 ℓ