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**superdude** There's a box with 16 cubic meter volume. Height z, depth y, width x. The sides cost $10/sq meter and top and bottom cost $20/sq meter. Write an equation for the cost (C) of the box using x and y.

I'm bad at coming up with formulas, is there a general procedure or any places where I should start?

I've got

sides=2*(z*y)*$10

front/back=2*(x*z)*10

top/bottom=2*(y*x)*20

and I think I should add them so cost=sides+front/back+top/bottom

This won't work so well as I have no clue how to incorporate the volume and I'm not allowed to use z.