Working Backwards from Weighted Average?

Hey there,

I'm working on allocating a unit mix for an apartment development and was looking for some suggestions on deciding unit sizes. In the attached file, my average weighted unit size is 1020 SqFt, im trying to figure out a way to input another average unit size, lets say 1300, and have the output show the corresponding unit sizes for each unit type to equal the weighted average of 1300, with respect to the unit mix. Any suggestions on how to do this?

Any help would be greatly appreciated.

Thanks