-4分之1+6分之5+3分之2-2分之1
=-1/4+5/6+2/3-1/2
=-1/4-1/2+5/6+2/3
=-3/4+3/2
=3/4
12-（-18)+（-7）-15
=12+18-7-15
=30-22
=8
4.7-(-8.9）-7.5+（-6）
=4.7+8.9-7.5-6
=4.6+9-13.5
=13.6-13.5
=0.1