As an optician, on average, you sell eyeglasses for $$80/$pair$.$ For every pair that you sell, the lenses, eyeglass frame, and the eyeglass case cost you altogether on average, $$55.$ Your total fixed expenses are $$500/$month$.$ For every pair of eyeglasses you sell, the contribution margin is: $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_.($calculate the contribution margin$).$ Be sure to put a $ in the front of your answer.