PMRevision of MA Test your understanding 3

I was going through the revision and i stuck on the question: The fastest time in which a batch of 20 ‘spicy meat special’ sandwiches has been made was 32 minutes, with no hold-ups. However, work studies have shown that, on average, about 8% of the sandwich makers’ time is non-productive and that, in addition to this, setup time (getting ingredients together etc.), is 2 minutes. If the sandwich-makers are paid $4.50 per hour, what is the attainable standard labour cost of one sandwich? I don't know how to work out the Non-productive idle time. any help? Thanks