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

	Scenario 1		Scenario 2
Tank type	Tank N	Tank P	Tank N	Tank P
Volume of the tank	3 u	7 u - 7	3 p	2 p x 3 = 6 p

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

3 u = 3 p --- (1)
7 u - 7 = 6 p
7 u = 6 p + 7 --- (2)

Make u the same.

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

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

(3) = (4)
21 p = 18 p + 21
21 p - 18 p = 21
3 p = 21
1 p = 21 ÷ 3 = 7

Volume of Tank P
= 6 p
= 6 x 7
= 42 ℓ

Answer(s): 42 ℓ