A colleague gave me a version of this that was fully scaffolded, but I thought it would be much richer if it was more open. And I threw a couple of tweep's names in there for fun.
They worked in groups and wrote solutions on their whiteboards. Here was the first attempt:
Although Fawn probably likes this, I asked if volume should determine the price. A. said "Surface area!" and this is what he produced:
And another solution:
Along the way I heard a lot of "Fawn's right!" and some "Nathan's wrong!" (maybe I should have won your little exponent game, Nathan)
They worked out that the volume of the large fish tank was 8 times the volume of the small one. The surface area of the large one was 4 times the surface area of the small one. The price of the large one was 4.6 times the price of the small one. Here is a nice summary (she originally forgot that there was no top so had to subtract after):
Bottom line here - Fawn is right! (always, otherwise she punches you)