简便计算:7.5*[63.9/(7-2.5)]

7.5*[63.9/(7-2.5)]
=7.5/7.5*｛[63.9/(7-2.5)]/7.5｝
=（63.9/4.5）/7.5
=（639/45）*4/（7.5*4）
=639/45*4/30
=639*4/（45*30）
=213*4/（45*10）
=213/45*2.5
.