1. Since the critical angle is (which is a little bit low in my opinion) you use a refractive index of .
2. Draw a sketch. (Attention: My sketch is not to scale!)
3. If the ray enters the glas under an angle of then the ray includes an angle of 40° with the perpendicular. Using Snell's law (have a look here: Snell's law - Wikipedia, the free encyclopedia) you'll get: