[iambrj]

I think a good example would be Lagrange's theorem[0]. Once you have this[1] picture in mind, it becomes trivial.

[0] https://en.wikipedia.org/wiki/Lagrange%27s_theorem

[1] https://i.stack.imgur.com/w2GfA.png

[sah2ed]

Was it this theorem of Lagrange's that you had in mind https://en.wikipedia.org/wiki/Lagrange%27s_theorem_(group_th... ?