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If we divide in the following way (why MathJax \cancel doesn't work on this page?): $$\require{cancel} \frac{1\cancel{\color{red}{6}}}{\cancel{\color{red}{6}}4}=\frac{1}{4}\qquad\text { ``dividing'' by 6} $$ we obtain the correct result. Another example is $$ \frac{1\color{red}{6}\color{cyan}{3}}{\color{cyan}{3}2\color{red}{6}}=\frac12. $$ Is there a method of finding more such examples?

Edit: I wish to thank @amWhy for the immediate help. I am leaving the unchanged question to future users with the same problem.

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    $\begingroup$ You need to declare it before using it (only one such declaration needed: $\require{cancel}$. $\endgroup$ – Namaste Jul 16 '18 at 15:29
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    $\begingroup$ @amWhy Is \require for a package or a command? (For the future usage). $\endgroup$ – Przemysław Scherwentke Jul 16 '18 at 15:31
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    $\begingroup$ Any fraction that includes two multiples of $10$ work if you take away the $0$'s. $\endgroup$ – RayDansh Jul 16 '18 at 15:32
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    $\begingroup$ \require requires a package. Of course it only works for some packages. $\endgroup$ – Daniel Fischer Jul 16 '18 at 15:37
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    $\begingroup$ This is called "Anomalous Cancellation". Relevant pages: oeis.org/A159975 mathworld.wolfram.com/AnomalousCancellation.html and benvitalenum3ers.wordpress.com/2012/03/17/… (the last one just because it gave me the term) $\endgroup$ – barrycarter Jul 16 '18 at 15:38

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