The total mass of 2 boxes and 3 tanks was 13.6 kg.
The total mass of 6 boxes and 6 tanks was 30.6 kg.

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

(a)
2 boxes + 3 tanks = 13.6 --- (1)
6 boxes + 6 tanks = 30.6 --- (2)

(2) _{÷ 6}
1 box + 1 tank = 5.1 --- (3)

Mass of 1 box and 1 tank = 5.1 kg

(b)
(3) x 2
2 boxes + 2 tanks = 2 x 5.1
2 boxes + 2 tanks = 10.2 --- (4)

(1) - (4)
(2 boxes + 3 tanks) - (2 boxes + 2 tanks) = 13.6 - 10.2
2 boxes - 2 boxes + 3 tanks - 2 tanks = 3.4

1 tank = 3.4

9 tanks = 3.4 x 9 = 30.6

Mass of 9 tanks = 30.6 kg

(c)
Substitute 1 tank = 3.4 into (3).
1 box + 1 tank = 5.1
1 box + 3.4 = 5.1
1 box = 5.1 - 3.4 = 1.7

Mass of 1 box = 1.7 kg

Mass of 1 box less than 1 tank
= 3.4 - 1.7
= 1.7 kg

Answer(s): (a) 5.1 kg; (b) 30.6 kg; (c) 1.7 kg