Thread: sample test question

My teacher sent out a sample test in preparation for tomorrows actual test. He says he will just "massage" the numbers.
Here is one question that has me flustered.

A rancher has 60 feet of fence to make a rectangular pen using a
long existing stone wall as one side. What is the maximum area that can
be made?

I was trying to solve by trial and error, but that isn't working.

The fence will make up 3 sides of the pen; follow that?
Let a = each of the 2 sides perpendicular to the stone wall:
then 60 - 2a is length of each of the other 2 sides: OK?

So Area = a(60 - 2a) = 60a - 2a^2
Maximize that!