a)

$2,84+7,9=10,74$

$\begin{array}{r}\stackrel{1}{2},84\\ \underline{+7,90}\\ 10,74\end{array}$

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b)

$20,01-8,63=11,38$

$\begin{array}{r}\stackrel{1}{\cancel{2}}\stackrel{9}{\cancel{0}},\stackrel{9}{\cancel{0}}\stackrel{11}{\cancel{1}}\\ \underline{-8,63}\\ 11,38\end{array}$

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c)

$-9,22\cdot \left(-8,5\right)=9,22\cdot 8,5=78,37$

$\begin{array}{r}\stackrel{1}{\stackrel{1}{9}},\stackrel{1}{\stackrel{1}{2}}2\\ \underline{\cdot 8,5}\\ 4610\\ \underline{+7376}\\ 78,370\end{array}$

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d)

$52,875:\left(-1,25\right)=-5287,5:125=-42,3$

$\begin{array}{l}\underline{42,3}\\ 5287,5:125\\ \underline{-500}\\ 287\\ \underline{-250}\\ 375\\ \underline{-375}\\ 0\end{array}$