Subtract Using Random Arrays

Heres my most current code. I have it where, onClick on an image it calls this. and it works, but the hp keeps restarting, like they all minus from 100, instead of the number that prieviously minused from 100. For xp: 100-9=91, is the first, the second, 100-72=28 instead of 91-72. How do i fix

var myhp;
function takehits()
{
var dmg= new Array()
dmg[0]=9;
dmg[1]=11;
dmg[2]=15;
dmg[3]=21;
dmg[4]=29;

myhp=100;
var rndtake=Math.floor(Math.random()*5);

document.getElementById('health').innerHTML=myhp-dmg[rndtake];
}
I want to have it where, you have an aray, and u have a number value that you want the number value, to be subtracted by one of the numbers in the array. And u lets say the array has 5 choices. Code: