A proofreader can read 40 pages in one hour. How many pages can this proofreader read in 90 minutes?
One hour is equal to 60 minutes. That gives you a fraction of 40 pages/60 minutes.
You don't know how many pages they can read in 90 minutes, so you would do
x pages/90 minutes.
The equation you would set up is 40/60 = x/90.
After cross multiplying you get 60x = 3600. Divide both sides by 60 to isloate the x, and you end up with x = 60.
A proofreader who can read 40 pages in one hour (60 minutes) can reading 60 pages in 90 minutes.