68-7.5+32-2.5
=68-[7.5＋2.5]＋32
=68＋32-10
=100-10
=90
7.8*10.1
=7.8×[10＋0.1]
=7.8×10＋7.8×0.1
=78＋0.78
=78.78
希望可以给你帮助