You must have an answer in the same book in which you found the question, and so in future you should ask about whatever it is in the answer that you are not clear about – then I will help you.
The overhead absorption rate is 900,000/60,000 = 15 per machine hour.
I do suggest that you watch my free lectures. They are a complete free course for Paper F2 and cover everything needed to be able to pass the exam well.
(This question is much too simple to be asked in Paper F2!)