If you draw a diagram of the triangle, you see that one has symmetry only if the segment of the ray's path within the prism is parallel to one side of the prism (the side through which it doesn't pass), leading to an angle of refraction of 30°. Thus, we use Snell's law (with the refractive index of air being approximated as 1):

Your calculator should give you the answer to that.

--Kevin C.