In order to derive the base of a triangle from its area, you need its height as well.
In fact, if we solve the area formula for the base, we have

So, the base length would be

where h is the height relative to the base you're interested in.
100$ because the income you have to earn is 400$, right? If so then 100 * 4 give you 400.
If you stay in amber city you waste $295 but if you go to bear lake your wasting around $795 so your saving $500
The price per cubic inch for the extra large tub will probably be 2000