basic algebra with several variables-why won't it work?

edit

You are using "implicit multiplication" which Sage does not understand by default (you can turn it on if you want). You should change 2y above to 2*y and 80(3200-x-y) to 80*(3200-x-y), etc... Here is code that works:

x, y = var('x, y')
solve([20*(6400-x-2*y)==0, 80*(3200-x-y)==0], x, y)

Sage's output is:

[[x == 0, y == 3200]]

ps. Implicit multiplication can be turned on using: implicit_multiplication(True), see the preparser documentation.