So I want this *bonus* to move accross the screen if the score is a multiple of 500,
bonus_mc.onEnterFrame = function(){
if(score_var ){
this._x +=bonus_speed;
if (this._x > Stage.width)
{
this._y = random(Stage.height/3);
this._x = -20;
}
}
}
Use the modulus operator to see if a value is a multiple of a number. If the result is 0 you have a multiple of the value. Since you specified two different values, your choices are...
if(score_var % 10 == 0){
or
if(score_var % 500 == 0){
And since a score_var of 0 would also produce a 0 result, if you do not want that included, then build it into the conditional as well.
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