Okay, so Tori has 16 pages now, and completes 5 an hour, so her equation would look something like P=5h+16, if we let P be total pages and h be pages per hour.
Now Cora has 13 pages and does 8 per hour. So Cora’s could be P=8h+13.
Now for the substitution bit, I guess we can substitute the P in the first equation, with what P equals in the second. In other words, (P) = 5h+16 would be
8h+13 = 5h+16. Subtract 5h from both sides to get 3h+13 = 16, and subtract 13 from both sides to get 3h = 3. Now divide by three and we get h=1, or in one hour they will be the same. To get pages, just replace the h in the first equations with one (they both will have 21 pages).
I hope this answer helps! :)