奥数简单操作有奖励。

=(13/4+20/3+7/4+25/3)×(2-7/20)=(13/4+7/4+25/3+20/3)×(2-7/20)=20×(2-7/20)=40-7=33

4 1/5× 25+32 1/7 4+0.25× 125

=(4+1/5)×25+(32+4/7)/4+1/4×125=100+5+8+1/7+125/4=113+1/7+31+1/4=144+11/28

(1×2×3+2×4×6+7×14×21)/(1×3×5+2×6×10+7×21×35)

=[(1×2×3)+(1×2×3)×2+(1×2×3)×7]/[(1×3×5)+(1×3×5)×2+(1×3×5)×7]

={(1×2×3)×[1+2+7]}/{(1×3×5)×[1+2+7]}=(1×2×3)/(1×3×5)=2/5