Exercises: addition of natural numbers

Add the following numbers by written addition.

$66+91$

For this task you need the following basic knowledge: Written method of addition
$\def\arraystretch{1.25} \begin{array}{l}\;\;\;\;\;\;\;6\;6\\\underline{+~~_1\;9\;1}\\\;\;\;\;1\;5\;7\end{array}$
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For this task you need the following basic knowledge: Written method of addition
$\def\arraystretch{1.25} \begin{array}{l}\;\;\;\;\;\;\;4\;\;\;2\\\underline{+~~_1\;7_1~\;8}\\\;\;\;\;1\;2\;\;\;0\end{array}$
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For this task you need the following basic knowledge: Written method of addition
$\def\arraystretch{1.25} \begin{array}{l}\;\;\;\;\;\;\;6\;3\\\underline{+~~_1\;8\;0}\\\;\;\;\;1\;4\;3\end{array}$
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