In math, words can mean different operations. A lot of times "of" means "multiplication". In this case he read 1/2 OF a 136 page book. So:
1/2 * 136 = 68 pages
Another way you can solve it is when we are talking about multiplying by a half, we can also just divide by 2 instead.
136 / 2 = 68 pages
I hope this explanation was thorough and helped you gain understanding on this subject! If you have any questions please don't hesitate to reach out!