The question tells you that the general overheads are $900,000 and that these are to be absurd on a labour hour basis. It also says that the total labour hours are 300,000. Therefore the overheads per hour are $900,000/300,000 = $3 per hour.
You are told that ZT3 takes 10 minutes of labour. Given there are 60 minutes in a hour, each unit takes 10/60 = 1/6 hours.