Hi, I have been struggling with this for ages, can you help with the creation of this formula ?!

What I want to be able to calculate is what A1 would contain if the % (in A4) was the figure in A5 and not A4.

cell A1: 100
cell A2: 400
cell A3: 500 (ie sum(A1:A2)
cell A4: 0.2 (or 20%. ie:sum(a1/a3)
cell A5: 0.15 (independent cell, fixed value)

So, I will be saying, if we were able to achieve the % in A5 then we would gain / loose x in A1.

Thanks for your help
Amy xx