7.2+(-1.8)-(-0.8)
=7.2-1.8+0.8
=7.2-(1.8-0.8)
=7.2-1
=6.2

0.6-1/4-（-3.75）+2/5
=0.6-0.25+3.75+0.4
=(0.6+0.4)+(3.75-0.25)
=1+3.5
=4.5

（-2/5）+（3又4/7）-1.6-（11/7）
=-(0.4+1.6)+(3又1/7-1又1/7)
=-2+2
=0