W każdym przykładzie szukana liczba została oznaczona jako $x$. 

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a)

$10\%\cdot x=30$

$0,1\cdot x=30\mid :0,1$  

$x=30:0,1=300:1=300$

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b)

$30\%\cdot x=150$

$0,3\cdot x=150\mid :0,3$   

$x=150:0,3=1500:3=500$ 

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c)

$45\%\cdot x=990$

$\frac{45}{100}\cdot x=990$  

$\frac{9}{20}\cdot x=9900\mid \cdot \frac{20}{9}$

$x={\sout{9900}}^{1100}\cdot \frac{20}{{\sout{9}}^1}=2200$

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d)

$240\%\cdot x=192$

$\frac{240}{100}\cdot x=192$

$\frac{12}{5}\cdot x=192\mid \cdot \frac{5}{12}$  

$x={\sout{192}}^{16}\cdot \frac{5}{{\sout{12}}^1}=80$

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e)

$350\%\cdot x=420$

$\frac{350}{100}\cdot x=420$  

$\frac{7}{2}\cdot x=420\mid \cdot \frac{2}{7}$   

$x={\sout{420}}^{60}\cdot \frac{2}{{\sout{7}}^1}=120$

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f)

$10,5\%\cdot x=84$

$0,105\cdot x=84\mid :0,105$   

$x=84:0,105=84000:105=\frac{84000}{105}=\frac{4000}{5}=800$

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g)

$2,8\%\cdot x=210$

$0,028\cdot x=210\mid :0,028$   

$x=210:0,028=210000:28=\frac{210000}{28}=\frac{30000}{4}=7500$

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h)

$0,02\%\cdot x=12$

$\frac{0,02}{100}\cdot x=12\mid \cdot 100$  

$0,02\cdot x=1200\mid :0,02$

$x=1200:0,02=120000:2=60000$

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i)

$125,4\%\cdot x=2508$

$\frac{125,4}{100}\cdot x=2508\mid \cdot \frac{100}{125,4}$  

$x=2508\cdot \frac{100}{125,4}={\sout{2508}}^2\cdot \frac{1000}{{\sout{1254}}^1}=2000$