We need to calculate the number of 3'' x 3'' square brownies can fit into a standard sheet pan of 18'' x 24''.
First, we must verify if a whole number of brownies can be fit into each dimension of the pan.
In 18'' we can fit 6 x 3'' brownies
In 24'' we can fit 8 x 3'' brownies.
Thus, the number of brownies is 6 * 8 = 48
We can cut 48 brownies from the standard sheet pan
Note if the dimensions of the pan would have been different, we could possibly not fit a whole number of brownies. For example, if the pan was 20'' x 25'', there is no way to fit 3'' square brownies. Some space must have been left over