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=7.5×2.1+7.8×7.9=7.5×2.1+（7.5+0.3）×7.9=7.5×2.1+7.5×7.9+0.3×7.9=7.5×（2.1+7.9）+2.37=7.5×10+2.37=75+2.37=77.37
