This answer is incorrect.

draw the correct model first, then it is easier to see which quantity need to add, which quantity need to subtract.

apologies for not being able to draw model in this comment.

Method 1
if consider Alan as 1 unit, then …
Benson = 1 unit - 2.2
Charles = 1 unit - 0.2

3 units = 118.2 + 2.2 + 0.2
=120.6
1 unit = 120.6 / 3 = 40.2
Benson = 1 unit - 2.2
= 40.2 - 2.2
= 38

Benson is 38 kg

Method 2
if consider Benson as 1 unit, then ...
Alan = 1 unit +2.2
Charles = Alan - 0.2
= 1 unit + 2.2 - 0.2
= 1 unit + 2.0

3 units = 118.2 - 2.2 - 2.0
= 114
1 unit = 114 / 3 = 38
Benson = 1 unit = 38 kg (as before)

checking ...
Alan = 40.2
Charles = 40
Total = 40.2 + 38 + 40 = 118.2 (correct!)