javascript - how to calculate the number to increment the variable -

how can calculate missing number, when add variable result equal or greater other variable.

• promoter: 10
• detractor: 2
• total: 12
• average: 66.67
• target: 75
• you need ??? promoters reach target.

i want find how many promoters need if average less target. how can calculate missing number when add promoter results of average equal or greater target.

thank you!

function missingnum() {  var xpromoter = ''; var x = 10; var y = 2; var target = 75; var z = x + y; var v = ((x - y) / z) * 100;      average = math.round(v * 100) / 100;          if (average<target) //how increment x average => target              document.write("promoter:" + "\n" + x + "<br>");              document.write("detractor:" + "\n" + y + "<br>" );               document.write("total:" + "\n" + z + "<br>" );              document.write("average:" + "\n" + average + "<br>" );              document.write("target:" + "\n" + target + "<br>" );              document.write("you need " + "\n" + xpromoter + "promoters reach target." );  } 

you make simple function.. calculation might better..

function calculateneededpromoters (promo, detrac) {     var percentage = 0.00;     var currentpromo = promo;     while (percentage < 75) {         currentpromo++;          var total = currentpromo + detrac;         var v = ((currentpromo - detrac) / total) * 100;         percentage = math.round(v * 100) / 100;     }     return currentpromo; }