The total mass of 3 tanks and 4 boxes was 15.6 kg.
The total mass of 4 tanks and 4 boxes was 17.2 kg.

1. What is the mass of 1 tank and 1 box?
2. What is the mass of 9 boxes?
3. How much less is the mass of 1 box than 1 tank?

(a)
3 tanks + 4 boxes = 15.6 --- (1)
4 tanks + 4 boxes = 17.2 --- (2)

(2) _{÷ 4}
1 tank + 1 box = 4.3 --- (3)

Mass of 1 tank and 1 box = 4.3 kg

(b)
(3) x 3
3 tanks + 3 boxes = 3 x 4.3
3 tanks + 3 boxes = 12.9 --- (4)

(1) - (4)
(3 tanks + 4 tanks) - (3 tanks + 3 tanks) = 15.6 - 12.9
3 tanks - 3 tanks + 4 boxes - 3 boxes = 2.7

1 box = 2.7

9 boxes = 2.7 x 9 = 24.3

Mass of 9 boxes = 24.3 kg

(c)
Substitute 1 box = 2.7 into (3).
1 tank + 1 box = 4.3
1 tank + 2.7 = 4.3
1 tank = 4.3 - 2.7 = 1.6

Mass of 1 tank = 1.6 kg

Mass of 1 tank less than 1 box
= 2.7 - 1.6
= 1.1 kg

Answer(s): (a) 4.3 kg; (b) 24.3 kg; (c) 1.1 kg