1：   

2.78+0.8+0.2

=2.78+﹙0.8+0.2﹚

=2.78+1

=3.78

0.37+1.79+0.63

=0.37+0.63+1.79

=1+1.79

=2.79

2：   

17.4+18.8+18.6+17.2

=﹙17.4+18.6﹚＋﹙18.8+17.2﹚

=36+36

=72秒

1：   

2.78+0.8+0.2

=2.78+﹙0.8+0.2﹚

=2.78+1

=3.78

0.37+1.79+0.63

=0.37+0.63+1.79

=1+1.79

=2.79

2：   

17.4+18.8+18.6+17.2

=﹙17.4+18.6﹚＋﹙18.8+17.2﹚

=36+36

=72秒