79×15+85×21
=(100-21)×15+85×21
=1500-21×15+85×21
=1500+21×(85-15)
=1500+21×70
=1500+1470
=2970
解:79*15+85*21
=79*15+(70+15)*21
=15*(79+21)+70*21
=15*100+1470
=1500+1470
=2970
79×15+85×21
=(80-1)×15+85×(20+1)
=1200-15+1700+85
=2970
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