8.15-3.78-2.22
=8.15-（3.78+2.22）
=8.15-6
=2.15
64+7.5+2.5
=64+（7.5+2.5）
=64+10
=74
4.76+5.87-2.76
=4.76-2.76+5.87
=2+5.87
=7.87