I was watching a video on combinatorics and the lecturer presented the original answer and the factored(simplified) answer.
The original answer:
The factorized answer:
I couldnt understand how did the author factorized the original answer so I went on to expand the simplified equation and the equation which I gotten was different from the original answer.
I realised that my expanded equation is missing $+ 10 * 4$, where did I got my steps wrong?