For this, we need to draw a model diagram. However, I am unable to draw it here, so I use 1 unit to represent 1 box of a model.

Now, we let Bob have a mass of 1 unit.

Then, Fred will have a mass of 2 units, because Fred is twice as heavy as Bob.

Alan is 8 kg heavier than Bob, so he has a mass of (1 unit + 8).

Let me attempt to draw the model here, where ### represents one unit of the model.

### + 8 for Alan
### for Bob
### ### for Fred

(Not sure if you are able to see this)

So, in total there are 4 units (ie 4 groups of ###) plus 8 more kg.

This total is 144 kg, so

4 units + 8 = 144

Removing 8 from the model gives

4 units = 144 - 8
4 units = 136

and then we find 1 unit by dividing our result by 4.

1 unit = 136 divided by 4 = 34

Alan’s mass is 1 unit + 8, so Alan’s mass
= 34 + 8
= 42 kg