desired total = base salary + commission
in this case, the commission will be based on x dollars times 2.5%
let's change 2.5% to a decimal, which is 0.025
to find x, we have to take the desired value, and divide it by the commission rate
so $200 ÷ 0.025, which is $8,000
so, if she sells $8,000, then her commission will be $200, which when added to her base salary will be $500