A regular polygon has $n$ sides . When the number of sides is doubled, each interior angle increases by $20^{\circ}$. Find $n$.
My workings till I got stuck
$1$ int. angle of $n$ sides $=180^{\circ}n-360^{\circ}/n$
$1$ int. angle of $2n$ sides $=200^{\circ}n-360^{\circ}/n$
$1$ ext. angle of $2n$ sides $=-20^{\circ}n+360^{\circ}/n$
$n=360$ divide $1$ ext. angle ...
I've done it till $-20n+360=360$
Then I got stuck. Can I get help? Thanks in advance!