[tex]\bf 24~~,~~\stackrel{24+4}{28}~~,~~\stackrel{28+4}{32}.....\qquad \stackrel{\textit{common difference}}{4} \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ n^{th}\textit{ term of an arithmetic sequence} \\\\ a_n=a_1+(n-1)d\qquad \begin{cases} n=n^{th}\ term\\ a_1=\textit{first term's value}\\ d=\textit{common difference}\\[-0.5em] \hrulefill\\ a_1=24\\ d=4\\ n=20 \end{cases} \\\\\\ a_{20}=24+(20-1)4\implies a_{20}=24+76\implies a_{20}=100[/tex]
