if he started reading from where he left off, and it's 5:30 and he is at page 160, and then he got to page 280 by 8:00, that is 2 hours and 30 minutes. so bring the reading time down into easier numbers so that 5-> 30 minute terms, then find out how many pages he read in 30 minutes. The difference between 160 pages and 280 pages is 120 pages. 120 divided by 30 equals 4. the amount of time between 3:30 and 5:30 is 2 hours which is 4-> 30 minute terms. So if it takes 30 minutes to read 4 pages then multiply 4 with 4 which equals 16, and finally subtract 16 from 160 and that's going to equal 144 pages.
(Sorry about the paragraph but i figured ide give you my reasoning and work just in case if i was wrong, ide like to know what i did if i am wrong. And if I'm right then that's how you show your work!)