7 tanks and 6 containers can hold 67.6 litres of water.
1 tank and 1 container can hold 9.9 litres of water.

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

(a)
7 tanks + 6 containers = 67.6 --- (1)
1 tank + 1 container = 9.9 --- (2)

(2) x 6
1 tank + 1 container = 9.9
6 tanks + 6 containers = 9.9 x 6
6 tanks + 6 containers = 59.4 --- (3)

(1) - (3)
(7 tanks + 6 containers) - (6 tanks + 6 containers) = 67.6 - 59.4
7 tanks + 6 containers - 6 tanks - 6 containers = 8.2
7 tanks - 6 tanks + 6 containers - 6 containers = 8.2

1 tank = 8.2

Volume of 1 tank = 8.2 ℓ

(b)
Substitute 1 tank = 8.2 into (2).
1 tank + 1 container = 9.9
8.2 + 1 container = 9.9

1 container = 9.9 - 8.2 = 1.7

Volume of 1 container = 1.7 ℓ

(c)
Volume of 7 containers
= 7 x 1.7
= 11.9 ℓ

(d)
Volume of 7 containers and 7 tanks
= 7 x 1.7 + 7 x 8.2
= 11.9 + 57.4
= 69.3 ℓ

Answer(s): (a) 8.2 ℓ; (b) 1.7 ℓ; (c) 11.9 ℓ; (d) 69.3 ℓ