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.