-4.2+5.7-8.4+10.2 =-(4.2+8.4)+(5.7+10.2)=-12.6+15.9=3.3
（-4.2+10.2）+（-8.4+5.7）
－4.2＋5.7－8.4＋10.2=10.2-4.2+5.7-8.4=6+5.7-8.4=11.7-8.4=3.3
