Leah has 2 containers. Container C and Container D are of different capacities. If Container C is filled by a tap at a rate of 4 litres per minute and Container D is filled by a tap at a rate of 7 litres per minute, when Container C is completely filled, 7 litres of water flowed out from Container D. If Container C is filled by a tap at a rate of 3 litres per minute and Container D is filled by a tap at a rate of 2 litres per minute, when Container C is completely filled, Container D is only half-filled. What is the capacity of Container D? Express your answer in litres.

	Scenario 1		Scenario 2
Container type	Container C	Container D	Container C	Container D
Volume of the container	4 u	7 u - 7	3 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 = 3 p --- (1)
7 u - 7 = 4 p
7 u = 4 p + 7 --- (2)

Make u the same.

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

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

(3) = (4)
21 p = 16 p + 28
21 p - 16 p = 28
5 p = 28
1 p = 28 ÷ 5 = 5.6

Volume of Container D
= 4 p
= 4 x 5.6
= 22.4 ℓ

Answer(s): 22.4 ℓ