They obviously know you will come up with the right answer. Its a question any ten year old wull be able to answer, but this is not questioning a ten year old but an interview at pwc. technically your answer should be 64. But if you think about it the answer can be what ever you want it to be. you can right down 22, 99,0.8373298. so when answering this question all you simply have to say is 'the answer can be whatever you want it to be'