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

	Scenario 1		Scenario 2
Tank type	Tank B	Tank C	Tank B	Tank C
Volume of the tank	3 u	7 u - 6	6 p	3 p x 3 = 9 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 - 6 = 9 p
7 u = 9 p + 6 --- (2)

Make u the same.

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

(2) x 3
21 u = 27 p + 18 --- (4)

(3) = (4)
42 p = 27 p + 18
42 p - 27 p = 18
15 p = 18
1 p = 18 ÷ 15 = 1.2

Volume of Tank C
= 9 p
= 9 x 1.2
= 10.8 ℓ

Answer(s): 10.8 ℓ