Each exterior angle measures 15°.

How to see it

For any regular polygon, the sum of all exterior angles is 360°.

If the polygon has 24 equal exterior angles, then each one is

exterior angle=360∘24=15∘\text{exterior angle}=\frac{360^\circ}{24}=15^\circ exterior angle=24360∘​=15∘

So a regular 24-sided polygon has exterior angles of 15° each.