You want to calculate the "rate" "pages per hour." To do this, divide "1/3 of a page" by "1/4 of an hour:"
(1 page)
------------
3
--------------------
(1 hr )
-----------
4
Remember: To divide by a fraction, invert the fraction and multiply instead.
Thus, you get
(1 page) 4 4 pages
------------ * ------------ = --------------- = (4/3) pages/hr
3 (1 hr) 3 hours
