7.4×99+7.8=7.4×99+(7.4+0.4)=7.4×(99+1)+0.4=7.4×100+0.4=740+0.4=740.4
=7.4×(100-1)+7.8=7.4×100-7.4×1+7.8=740+(7.8-7.4)=740+0.4=740.4
7.4*(100-1)+7.8=740+7.8-7.4=740+0.4=740.4
7.4x(100-1)+7.8=740-7.4+7.8=740+0.4=740.4
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