The four angles around a point are $x,x+20$°, $x+40$° and $x+60$°. Find $x$.

$4x+120$° = 360°, so $4x=240$° and $x=60$°. The angles are 60°, 80°, 100°, 120° (check: 360°). (90° comes from forgetting the extra 120°.)