Question 834665
{{{s(n)=0.04n+2500}}}

Let i(n) be the inverse of s(n).


{{{s(i(n))=0.04(i(n))+2500=n}}};
Solve for i(n).
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{{{0.04*i(n)=n-2500}}}
{{{i(n)=(n-2500)/0.04}}}


The way this is asked, you are given s(n) and are asked to find n.  Use of i(n) seems inapplicable.  I'd start with just the original function, {{{s(n)=2820=0.04n+2500}}}
and solve for n.
{{{(2820-2500)/0.04}}}, but this IS what was found to be the inverse expression.  s and n have appeared to switch purposes.  This amount of merchandise sales is 8000 dollars.