Breaking Instance Name


So for one of my project I am trying to call movieClips that are named in the series of png1_mc, png2_mc…png30_mc… I need to call these randomly when am instance is triggered

Something in the lines of

var mv63:Tween = new Tween([COLOR=Red]png+id_mc[/COLOR],"scaleX", Regular.easeInOut, 0, 1,0.5,true);

Where in png+id_mc, id is the string value of the integer I am randomly generating. Now I don’t know how to get this to work, as flash does not allow me to break the instance name like that. Any Idea how to get this done?