Got a numerical problem in optics, can u solve.. my teacher said its wrong.

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the angle of incident of light that passes through 60 degree prism is given as 45 degree. and the refractive index of prism is 1.5. find the angle of emergence and angle of deviation.. I applied the formula assuming the prism is very small angled. D=A[n-1] D is deviation, A is angle of prism and n is refractive index, to find the deviation i applied this formula but my teacher gave me 0 ,zero for this. why so? huh? isn't this a correct method? huh?