6 containers and 5 tanks can hold 39.1 litres of water.
1 container and 1 tank can hold 7.1 litres of water.

1. How many litres of water can 1 container hold?
2. How many litres of water can 1 tank hold?
3. How many litres of water can 7 tanks hold?
4. How many litres of water can 5 tanks and 6 containers?

(a)
6 containers + 5 tanks = 39.1 --- (1)
1 container + 1 tank = 7.1 --- (2)

(2) x 5
1 container + 1 tank = 7.1
5 containers + 5 tanks = 7.1 x 5
5 containers + 5 tanks = 35.5 --- (3)

(1) - (3)
(6 containers + 5 tanks) - (5 containers + 5 tanks) = 39.1 - 35.5
6 containers + 5 tanks - 5 containers - 5 tanks = 3.6
6 containers - 5 containers + 5 tanks - 5 tanks = 3.6

1 container = 3.6

Volume of 1 container = 3.6 ℓ

(b)
Substitute 1 container = 3.6 into (2).
1 container + 1 tank = 7.1
3.6 + 1 tank = 7.1

1 tank = 7.1 - 3.6 = 3.5

Volume of 1 tank = 3.5 ℓ

(c)
Volume of 7 tanks
= 7 x 3.5
= 24.5 ℓ

(d)
Volume of 5 tanks and 6 containers
= 5 x 3.5 + 6 x 3.6
= 17.5 + 21.6
= 39.1 ℓ

Answer(s): (a) 3.6 ℓ; (b) 3.5 ℓ; (c) 24.5 ℓ; (d) 39.1 ℓ