Ok so to start let's put everything we know in some expressions
Words: In September, John read a total of p pages.
September: p
Words: In October, he read twice as many pages than he read in September
October: 2p
Words: November, he read triple the number of pages that he read in October
November: 3 x October
Then, we know what October is (2p), so we can substitute it into the third expression to get 3 x 2p or 6p. In November, John read 6p pages, or 6 times as many as he read in September.
Note: The fact that he read 120 pages in December is just in there to distract you. It's never connected to any of the other months, and since our equation can be in terms of p we don't need to know the actual number of pages
If I can help with anything else just let me know :)