7 pails and 6 tanks can hold 44.1 litres of water.
1 pail and 1 tank can hold 6.8 litres of water.

1. How many litres of water can 1 pail hold?
2. How many litres of water can 1 tank hold?
3. How many litres of water can 9 tanks hold?
4. How many litres of water can 6 tanks and 7 pails?

(a)
7 pails + 6 tanks = 44.1 --- (1)
1 pail + 1 tank = 6.8 --- (2)

(2) x 6
1 pail + 1 tank = 6.8
6 pails + 6 tanks = 6.8 x 6
6 pails + 6 tanks = 40.8 --- (3)

(1) - (3)
(7 pails + 6 tanks) - (6 pails + 6 tanks) = 44.1 - 40.8
7 pails + 6 tanks - 6 pails - 6 tanks = 3.3
7 pails - 6 pails + 6 tanks - 6 tanks = 3.3

1 pail = 3.3

Volume of 1 pail = 3.3 ℓ

(b)
Substitute 1 pail = 3.3 into (2).
1 pail + 1 tank = 6.8
3.3 + 1 tank = 6.8

1 tank = 6.8 - 3.3 = 3.5

Volume of 1 tank = 3.5 ℓ

(c)
Volume of 9 tanks
= 9 x 3.5
= 31.5 ℓ

(d)
Volume of 6 tanks and 7 pails
= 6 x 3.5 + 7 x 3.3
= 21 + 23.1
= 44.1 ℓ

Answer(s): (a) 3.3 ℓ; (b) 3.5 ℓ; (c) 31.5 ℓ; (d) 44.1 ℓ