Relevant cost

Hi Dear Tutor, I have a question I took this example from Bpp study text "short-term decision" chapter Special job will require 20 hours of skilled labour. The skilled workforce is paid $15 per hour and is working at full capacity. If the special job is undertaken, the skilled labour would be taken off the work that earns a contribution of $12 per hour,after deducting the costs of the labour and variable overheads of $1.5 per hour. Labour cost of 20 hrs *15$ per hour=300 Variable overhead costs 20hrs * $1.50 per hour=30 Contribution forgone (lost contrubution) 20hrs * $12 per hour=240 total relevant cost======570 Ennerdale multiple choice question number 108 Labour The contract requires 800 hours of skilled labour. Skilled labour is paid $9.50 per hour. There is a shortage of skilled labour and all the available skilled labour is fully employed in the company in the manufacture of Product P Selling price per unit----100 Skilled labor cost per unit----38 Other variable cost per unit-22 Contrubution =40 Labour hout per unit=38/9.50=4 hours per unit Skilled labour ---800* $9.50 labour cost per hout=7600 Lost contrubution (40/4=10*800)============8000 relevant cost ========================15600 Variable cost per unit=====22 Direct labour hour per unit==4 variable cost per hour=22/4=5.5 variavle cost (800*5.5)-is not considered here? but above example as we can see that "Variable overhead costs 20hrs * $1.50 per hour=30" f the special job is undertaken, the skilled labour would be taken off the work that earns a contribution of $12 per hour,after deducting the costs of the labour and variable overheads of $1.5 per hour. it says contribution after labour and variable cost!