If $a$ and $b$are the coefficients of $x^n$ in the expansions of $(1+x{)}^{2n}$ and $(1+x{)}^{2n-1}$ respectively, find $\frac{a}{b}$

If in the expansion of $(1+x{)}^{20}$ the coefficients of $r^{\mathrm{th}}$ and $(r+4{)}^{\mathrm{th}}$ terms are equal, then $r$ is equal to

If in the expansion of $(a+b{)}^n\text{ and }(a+b{)}^{n+3}$ the ratio of the coefficients of second and third terms, and third and fourth terms respectively are equal, then $n$ is