-4.2+5.7-8.4+10=3.1
-1/4+5/6+2/3-1/2
=-3/12+10/12+8/12-6/12
=3/4
12+（-18）+（-7）-15
=12-18-7-15
=-28
4.7-（-8.9）-7.5+（-6）
=4.7+8.9-7.5-6
=0.1