（1）2.31×1.5+1.5×7.69

=（2.31+7.69）×1.5

=10×1.5

=15；

（2）23.4÷7.8-2.1

=3-2.1

=0.9；

（3）4.5×0.4÷4.5×0.4

=4.5÷4.5×0.4×0.4

=1×0.4×0.4

=0.16；

（4）6.4×101-6.4

=6.4×（101-1）

=6.4×100

=640；

（5）4×1.98×0.25

=4×0.25×1.98

=1×1.98

=1.98；

（6）7.5+2.5-7.5+2.5

=7.5-7.5+（2.5+2.5）

=0+5

=5．