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

	Scenario 1		Scenario 2
Tank type	Tank C	Tank D	Tank C	Tank D
Volume of the tank	5 u	7 u - 6	6 p	2 p x 4 = 8 p

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

5 u = 6 p --- (1)
7 u - 6 = 8 p
7 u = 8 p + 6 --- (2)

Make u the same.

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

(2) x 5
35 u = 40 p + 30 --- (4)

(3) = (4)
42 p = 40 p + 30
42 p - 40 p = 30
2 p = 30
1 p = 30 ÷ 2 = 15

Volume of Tank D
= 8 p
= 8 x 15
= 120 ℓ

Answer(s): 120 ℓ