[tex] \frac{81}{10} = \frac{9}{n} \\ \\ cross \: multiply: \\ 81n = 90 \\ \\ divide \: both \: sides \: by \: 9 :\\ 9n = 10 \\ \\\implies n = \boxed{\frac{10}{9}} [/tex]
[tex]\huge\mathfrak\colorbox{white}{}[/tex]
Want to know more? Know the Binomial Theorem here(because I have it pinned on my clipboard) --
[tex]\boxed{{(a+b)} ^n= \sum_{k=0}^{n}\binom{n}{k} a^{n-k}.b^{k}}[/tex]
