to find the scale factor, the main formula which is used for solving such a problem is as follow: If the initial shape is greater than the final's, so we use SF=SL / LL. SF= scale factor, SL=Smaller length, LL= Larger legth. According to our case, the smaller length is SL=6mm, and the Larger length is LL=15mm, so SF=6mm/15mm=2/0.4mm.The answer is SF=0.4mm