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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.

Przemysław Scherwentke
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    You need to declare it before using it (only one such declaration needed: $\require{cancel}$. – amWhy Jul 16 '18 at 15:29
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    @amWhy Is \require for a package or a command? (For the future usage). – Przemysław Scherwentke Jul 16 '18 at 15:31
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    Any fraction that includes two multiples of $10$ work if you take away the $0$'s. – RayDansh Jul 16 '18 at 15:32
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    \require requires a package. Of course it only works for some packages. – Daniel Fischer Jul 16 '18 at 15:37
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    This is called "Anomalous Cancellation". Relevant pages: https://oeis.org/A159975 http://mathworld.wolfram.com/AnomalousCancellation.html and https://benvitalenum3ers.wordpress.com/2012/03/17/fractions-anomalous-cancellation/ (the last one just because it gave me the term) –  Jul 16 '18 at 15:38
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    See https://math.stackexchange.com/questions/2011233/explain-how-and-why-the-cancellation-of-6-s-in-dfrac1664-to-get and https://math.stackexchange.com/questions/49657/bad-fraction-reduction-that-actually-works and https://math.stackexchange.com/questions/2512373/anomalous-cancellation-definition – cgiovanardi Jul 16 '18 at 21:53

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