help need a formula to figure how much more an employee can sell with more productivity

If sarah sells $10 per hr and makes $400 how much could she be making if she were to increase her productivity to selling $25 per hr. I need to make a spreadsheet listing each emplys productivity.Then showing how much they could be making if they sold more per hr.
I have no clue how to formulate this.

if you have a cell with with the amount per hour $10 say A2
and the total $400 say B2
then you can work out the hours

In a cell you have the new figure required ie $25 C2

then in D2
so the formula would be 25* (400/10) - 400 would give the result
= (C2*(B2/A2)) -B2
should work