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第39页

信息发布者：

 解：

 $\begin{aligned}4x + 0.8×6 &= 15.2\\4x + 4.8 &= 15.2\\4x &= 15.2 - 4.8\\4x &= 10.4\\x &= 2.6\end{aligned}$

解：$\frac {2}{3}x=\frac {5}{6}$

$x=\frac {5}{4}$

解：$\frac {5}{8}x=\frac {15}{2}$

$x=12$

解：$\frac {2}{3}x=\frac {2}{9}×21$

$\frac {2}{3}x=\frac {14}{3}$

$x=7$

 $\begin{aligned}772 - 317 + 128 - 283&=(772 + 128)-(317 + 283)\\&=900 - 600\\&=300\end{aligned}$

 $\begin{aligned}0.4×(12×0.25)&=(0.4×0.25)×12\\&=0.1×12\\&=1.2\end{aligned}$

 $\begin{aligned}(3.8 - 1.8÷2)×0.5&=(3.8 - 0.9)×0.5\\&=2.9×0.5\\&=1.45\end{aligned}$

 $\begin{aligned}(\frac{5}{12}+\frac{1}{19})×12 + \frac{7}{19}&=\frac{5}{12}×12 + \frac{1}{19}×12 + \frac{7}{19}\\&=5 + (\frac{12}{19}+\frac{7}{19})\\&=5 + 1\\&=6\end{aligned}$

 $\begin{aligned}127×0.75 - 26×\frac{3}{4} - 75\%&=(127 - 26 - 1)×0.75\\&=100×0.75\\&=75\end{aligned}$

 $\begin{aligned}\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}&=1 - \frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\\&=1 - \frac{1}{7}\\&=\frac{6}{7}\end{aligned}$