The total mass of 2 tanks and 3 boxes was 13.3 kg.
The total mass of 6 tanks and 6 boxes was 30 kg.

1. What is the mass of 1 tank and 1 box?
2. What is the mass of 7 boxes?
3. How much less is the mass of 1 box than 1 tank?

(a)
2 tanks + 3 boxes = 13.3 --- (1)
6 tanks + 6 boxes = 30 --- (2)

(2) _{÷ 6}
1 tank + 1 box = 5 --- (3)

Mass of 1 tank and 1 box = 5 kg

(b)
(3) x 2
2 tanks + 2 boxes = 2 x 5
2 tanks + 2 boxes = 10 --- (4)

(1) - (4)
(2 tanks + 3 tanks) - (2 tanks + 2 tanks) = 13.3 - 10
2 tanks - 2 tanks + 3 boxes - 2 boxes = 3.3

1 box = 3.3

7 boxes = 3.3 x 7 = 23.1

Mass of 7 boxes = 23.1 kg

(c)
Substitute 1 box = 3.3 into (3).
1 tank + 1 box = 5
1 tank + 3.3 = 5
1 tank = 5 - 3.3 = 1.7

Mass of 1 tank = 1.7 kg

Mass of 1 tank less than 1 box
= 3.3 - 1.7
= 1.6 kg

Answer(s): (a) 5 kg; (b) 23.1 kg; (c) 1.6 kg