＝(30×2/3)×1/13＝20×1/13＝20/13＝1又7/13
=30*1/13*2/3=30*2/3*1/13=20*1/13=20/13=1又7/13≈1.54
30×1/13×2/3=30×2/3×1/13=20×1/13=20/13
