P1 – Relevant Costing (A BPP question)

Hi,

With the below question, I cannot figure out why $3 has been considered to calculate the relevant cost for the special order. Could you please explain why?

“In a manufacturing plant, the work force is operating at full capacity. The work force is paid a fixed rate of $12 per hour for a 35-hour week. This is a fixed weekly wage, although for the purpose of management accounting, direct labour costs are treated as a variable cost. A customer has asked for a special job to be done that would involve taking employees off regular work that earns a contribution of $15 per hour, after allowing for direct labour cost and variable overheads at $3 per hour. The company must decide whether to agree to the customer’s request for the work, and to set a price. The customers request is expected to take 10 hours” Relevant cost for labour for this job? Thank you