①7.8-1.9+8.2-8.1
=（7.8+8.2）-（1.9+8.1）
=16-10
=6
②0.45×78.2+20.8×0.45+0.45
=0.45×（78.2+20.8+1）
=0.45×100
=45
③99×7.4
=（100-1）×7.4
=100×7.4-1×7.4
=740-7.4
=732.6