6 tanks and 5 pails can hold 23.9 litres of water.
1 tank and 1 pail can hold 4.2 litres of water.

1. How many litres of water can 1 tank hold?
2. How many litres of water can 1 pail hold?
3. How many litres of water can 4 pails hold?
4. How many litres of water can 4 pails and 6 tanks?

(a)
6 tanks + 5 pails = 23.9 --- (1)
1 tank + 1 pail = 4.2 --- (2)

(2) x 5
1 tank + 1 pail = 4.2
5 tanks + 5 pails = 4.2 x 5
5 tanks + 5 pails = 21 --- (3)

(1) - (3)
(6 tanks + 5 pails) - (5 tanks + 5 pails) = 23.9 - 21
6 tanks + 5 pails - 5 tanks - 5 pails = 2.9
6 tanks - 5 tanks + 5 pails - 5 pails = 2.9

1 tank = 2.9

Volume of 1 tank = 2.9 ℓ

(b)
Substitute 1 tank = 2.9 into (2).
1 tank + 1 pail = 4.2
2.9 + 1 pail = 4.2

1 pail = 4.2 - 2.9 = 1.3

Volume of 1 pail = 1.3 ℓ

(c)
Volume of 4 pails
= 4 x 1.3
= 5.2 ℓ

(d)
Volume of 4 pails and 6 tanks
= 4 x 1.3 + 6 x 2.9
= 5.2 + 17.4
= 22.6 ℓ

Answer(s): (a) 2.9 ℓ; (b) 1.3 ℓ; (c) 5.2 ℓ; (d) 22.6 ℓ