Labour efficiency variance mock exam question

Hi, I do not understand the solution at all sir.

To calculate efficiency variance, I know that it is Actual hours * standard rate and you compare the answer to Standard hours * Standard Rate.

I think I do not know how to work out standard rate for labour.

Please could you explain.

The question is:

Company has budgeted labour cost of $180,000 for the production of 30,000 units per month. Each unit is budgeted to take 3 hours of labour. Actual cost is $160,000 for 28,000 and 85,000 hours were worked.

What is the labour efficiency variance?

Could you please explain how to get the standard rate because I still do not understand from the solution provided?

Many thanks