7加负9加负4加9减4加7减负20加负8

问题描述:

7加负9加负4加9减4加7减负20加负8

7+(-9)+(-4)+9-4+7-(-20)+(-8)
=7-9-4-4+9+7+20-8
=7+7+20+9-(9+4+4+8)
=43-25
=18