After writing part of his novel, thomas is now writing 16 pages per week. after 4 weeks, he has written 85 pages. write an equation to represent the number of pages he writes.

number of pages = 16w + 21
where w is the number of weeks

Explanation:
Thomas has decided to write 16 pages per week. This means that in 4 weeks, Thomas would write:
number of pages written in 4 weeks = 4*16 = 64 pages

We are given that after 4 weeks, Thomas had a total of 85 written pages.
This number is equal to the the summation of:
i- What he has written before deciding to write 16 pages per week
ii- The number of pages he wrote on the constant pace (64 pages)

Therefore:
85 = number of already written pages + 64
number of already written pages = 85 - 64 = 21 pages

Now, the equation that represents the pages he writes will be equal to the number of pages he writes in a certain number of weeks added to the 21 pages that were already written
Therefore:
equation is:
number of pages = 16w + 21
where w is the number of weeks

Hope this helps :)