Relevant costing

Labor
100 hours of skilled and 56 hours of unskilled labor will be required for the upholstery and
flooring work. Skilled labor is paid the market rate of $25/hour and is currently fully
employed on another job, where they earn a contribution of $6/hour. Alternatively new
skilled labor could be employed, but the new workers will require training at a cost of
$14/hour for the first 10 hours they are working.
Unskilled workers are currently paid $12/hour and each of the 5 workers is guaranteed a
minimum wage of $420 per week. Each unskilled employee has enough work allocated to
them for the next three weeks to earn $372 per week. The work to be done by the unskilled
labor on Bow must be completed within the first week of the project starting and overtime
is paid at time and half.
What cost should be included in the quote for the skilled and unskilled labour?
$_______ skilled labour $_______ unskilled labour

Skilled labour $2,640 Unskilled labour $648
The existing skilled labour can be used costing ($25 + $6) × 100 = $3,100
OR new labour can be hired and trained costing $25 × 100 + $14 ×10 = $2,640
Take the cheaper cost.
The unskilled workers are guaranteed work for $420/$12 = 35 hours.
They are currently working $372/$12 = 31 hours. Therefore there are 4 hours per week
per worker spare capacity. There are 5 workers therefore 20 hours a week will have
no cost, leaving 36 hours to be paid for at time and a half.
36 × $12 × 1.5 = $648

Sir please help me, I don’t understand how 36 hours to be paid for at time and a half was got