=(-0.8+0.8)+( 1.2+3.5-0.7)-2. 1

=4-2. 1

= 1.9

1/2+(-2/3)+4/5+(- 1/2)+(- 1/3)

=( 1/2- 1/2)+4/5-(2/3+ 1/3)

=4/5- 1

=- 1/5

-4.2+5.7-8.4+ 10

=(5.7+ 10)-(4.2+8.4)

= 15.7- 12.6

=3. 1

- 1/4+5/6+2/3- 1/2

=(5/6+2/3)-( 1/4+ 1/2)

=(5/6+4/6)-( 1/4+2/4)

=9/6-3/4

=3/2-3/4

=6/4-3/4

=3/4