Let $\frac { 1 } { x _ { 1 } } , \frac { 1 } { x _ { 2 } } , \ldots , \frac { 1 } { x _ { n } } \left( x _ { i } \neq 0 \right.$ for $\left. i = 1,2 , \ldots , n \right)$ be in A.P. such that $x _ { 1 } = 4$ and $x _ { 21 } = 20$. If $n$ is the least positive integer for which $x _ { n } > 50$, then $\sum _ { i = 1 } ^ { n } \left( \frac { 1 } { x _ { i } } \right)$ is equal to\\
(1) 3\\
(2) $\frac { 1 } { 8 }$\\
(3) $\frac { 13 } { 4 }$\\
(4) $\frac { 13 } { 8 }$