-4.2+5.7-8.4+10
=5.7-4.2+10-8.4
=1.5+1.6
=3.1
-1/4+5/6+2/3-1/2
=5/6+4/6-1/4-1/2
=9/12-3/4
=3/4-3/4
=0
12-（-18）+（-7）-15
=12-7+18-15
=5+3
=8
4.7-（-8.9）-7.5+（-6）
=4.7+8.9-7.5-6
=4.7+1.4-6
=6.1-6
=0.1