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?$解:設(shè)較長鐵棒的長度為\ x \mathrm{~cm} , 較短鐵棒的長度為\ y \mathrm{~cm} .\ $?
?$因為兩根鐵棒之和為\ 55 \mathrm{~cm} , 故\ x+y=55 .\ $?
?$又知兩棒未露出水面的長度相等, 故可知\ \frac{2}{3} x=\frac{4}{5} y .\ $?
?$于是\ \left\{\begin{array}{l}x+y=55, \\ \frac{2}{3} x=\frac{4}{5} y .\end{array}\right.\ 解得\ \left\{\begin{array}{l}x=30, \\ y=25 .\end{array}\right.\ $?
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?$\begin {cases}{x=2y+6}\\{y+9=2(x-9)}\end {cases}$?
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?$解:設(shè)這艘輪船在靜水中的速度和水流速度分別為\ x \mathrm{~km} / \mathrm{h}, y \mathrm{~km} / \mathrm{h} ,\ $?
?$則順水速度為\ (x+y) \mathrm{km} / \mathrm{h} , 逆水速度為\ (x-y) \mathrm{km} / \mathrm{h}\ ,\ $?
?$由題意知: \left\{\begin{array}{l}7(x+y)=140 \\10(x-y)=140\end{array}\right.$?
?$解得:\ \left\{\begin{array}{l}x=17 \\ y=3\end{array}\right.\ $?
?$\ \therefore\ 這艘輪船在靜水中的速度和水流速度分別為\ 17 \mathrm{~km} / \mathrm{h}, 3 \mathrm{~km} / \mathrm{h}\ $?
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?$解:設(shè)有汽車\ x\ 輛, 學(xué)生\ y\ 人.\ $?
?$根據(jù)題意, 得\ \left\{\begin{array}{l}45 x=y-15, \\ 60(x-1)=y .\end{array}\right.\ \ $?
?$解得\ x=5 .$?
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?$解:設(shè)王大爺種了\ x\ 畝茄子,\ y\ 畝西紅柿.\ $?
?$根據(jù)題意, 得\ \left\{\begin{array}{l}x+y=25, \\ 1700 x+1800 y=44000 .\end{array}\right.\ 解得\ \left\{\begin{array}{l}x=10, \\ y=15 .\end{array}\right.\ \ $?
?$因此, 共獲純利\ 2400 \times 10+\ \ 2600 \times 15=63000\ (元).\ $?
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