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A ray in air is incident on a glass plate whose index of refraction is 1.60. The angle of refraction is one half the angle of reflection. The angle of refraction is closest to:

I know to use snell's law and theta_a = theta_r but the problem doesn't give me any angles to compute theta_b

(1) sin (theta_a) = 1.60 sin (theta_b)

and

theta_b = 1/2 theta_r = 1/2 theta_a

I'm I suppose to assume something here?