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第58页

信息发布者：

5:8

5:13

3:17

1:4

25:157

$\begin {aligned}解：\frac {1}{4}x&=5×\frac {1}{5}\\\frac {1}{4}x&=1\\x &=4\end {aligned}$

$\begin {aligned}解：7x&=9×3\\7x&=27\\x &=\frac {27}{7}\end {aligned}$

$\begin {aligned}解：\frac {2}{5}x&=\frac {3}{4}×\frac {1}{2}\\\frac {2}{5}x&=\frac {3}{8}\\x &=\frac {15}{16}\end {aligned}$

$7.2×\frac 1{1+8}=0.8($千克)

$7.2-0.8=6.4($千克)

答：$7.2$千克水中含氢$0.8$千克，氧$6.4$千克。