Labour rate variance

Ring Co’ standard labour cost is based on 2 hours of semi-skilled labour at $20/hr. The labour supervisor arranged for the work to be done in pairs, using one semi skilled and one unskilled worker, each
working for 1.25 hours
What is the maximum hourly rate, to the nearest $0.01, that the supervisor can afford to pay the unskilled workers without giving rise to an adverse total labour variance?
Will you please assist me in this question.I’ll be thankful.