Solving for X (assuming if Statement

[montanaaggie]

A1=.2
A2=.3
A3=.5
A4=sum(A1:A3)=1.0

I'd like to make a calculation to put in A1 (at least I think its this cell), that would allow me to change A2, leave A3 constant at .5, and A4 sum would always be equal to 1.0. For example in the next scenario. A2=.4, A3=.5, and A4, the sum would still equal 1. What equation would be needed for A1 to equal .1?

Thanks,

 

[tweedle]

A1: =1-(sum(a2:a3))