7 containers and 6 tanks can hold 28.7 litres of water.
1 container and 1 tank can hold 4.4 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 3 tanks hold?
4. How many litres of water can 4 tanks and 7 containers?

(a)
7 containers + 6 tanks = 28.7 --- (1)
1 container + 1 tank = 4.4 --- (2)

(2) x 6
1 container + 1 tank = 4.4
6 containers + 6 tanks = 4.4 x 6
6 containers + 6 tanks = 26.4 --- (3)

(1) - (3)
(7 containers + 6 tanks) - (6 containers + 6 tanks) = 28.7 - 26.4
7 containers + 6 tanks - 6 containers - 6 tanks = 2.3
7 containers - 6 containers + 6 tanks - 6 tanks = 2.3

1 container = 2.3

Volume of 1 container = 2.3 ℓ

(b)
Substitute 1 container = 2.3 into (2).
1 container + 1 tank = 4.4
2.3 + 1 tank = 4.4

1 tank = 4.4 - 2.3 = 2.1

Volume of 1 tank = 2.1 ℓ

(c)
Volume of 3 tanks
= 3 x 2.1
= 6.3 ℓ

(d)
Volume of 4 tanks and 7 containers
= 4 x 2.1 + 7 x 2.3
= 8.4 + 16.1
= 24.5 ℓ

Answer(s): (a) 2.3 ℓ; (b) 2.1 ℓ; (c) 6.3 ℓ; (d) 24.5 ℓ