Reverse calculation formulas needed

[lro110]

Hi looking for help am sure it can be done but can't get my head around how

What i need to work out is the following, I want to input (e) answer and work out the start figure (a)

current cals is

a (475) *18%=b 85.50 b*20%=c 17.10 b+c=d a-d=e

I want to be able to input e and get A

Thanks in advance

[kev_]

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thanks

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