得到没有百分比的总和

Question

I work widht service and make comission. For exaple i do 3% comission + 15

$summ = 800;
$rez = ($summ + ($summ * 0.03)) + 15
//$rez returned 839

How i get back $summ (800) from $rez? I try this

$rez = 839;
$summ = ($summ - ($summ * 0.0285)) - 15;
//$summ returned 800.09

if i roud it, i take 800, but other nubmers not work

    800 (839) - (800.09)
    300 (324) - 276.45
    600 (633) - 567.9
    900 (942) - 859.35
    3590 (3712.7) - 3472.69
    4550 (4701.5) - 4405.33
    10000 (10315) - 9700
    9700 (10006) - 9408.55

Help please.

Answer 1

You have to solve your equation for $summ :

$rez  = ($summ + ($summ * 0.03)) + 15
      = 1.03 * $summ + 15
$summ = ($rez - 15) / 1.03

Answer 2

您应该通过以下等式返回它：

$summ =($rez-15)*3/100*800;