一列数按某规律排列如下:$\dfrac{1}{1}$,$\dfrac{1}{2}$,$\dfrac{2}{1}$,$\dfrac{1}{3}$,$\dfrac{2}{2}$,$\dfrac{3}{1}$,$\dfrac{1}{4}$,$\dfrac{2}{3}$,$\dfrac{3}{2}$,$\dfrac{4}{1}$,$\ldots$,若第$n$个数为$\dfrac{5}{7}$,则$n=\left( \right)$