1.7+(-2)-3.4
=7-2-3.4
=5-3.4
=1.6
2.-21.6+3-7.4+(-2/5)
=(-21.6-7.4)+3-2/5
=-29+3-2/5
=-26又五分之二
3.31+(-5/4)+0.25
=31-1.25+0.25
=31-1
=30
4.7-(-1/2)+1.5
=7+0.5+1.5
=7+2
=9
5.49-(-20.6)-3/5
=49+20.6-0.6
=49+20
=69
6.(-6/5)-7 -(-3.2)+(-1)
=-1.2-7+3.2-1
=(-1.2+3.2)-8
=2-8
=-6