need help with this question part a)

After spilling his plate of spaghetti all over his friend's apartment floor, Kevin rented a carpet cleaner to clean it up. In doing so, he realized just how easy it is to clean carpets! He started his own business, diving in head-first by purchasing a carpet cleaner, to make a little money. His friends are keen to use his services to clean their rented apartments before handing back keys to the landlords. Kevin charges $80 to clean the main areas of an apartment. In addition to his equipment costs, he uses a cleaning solution on the carpet, which costs about $2 per unit cleaned. Because of his class load, Kevin currently cleans just 4 apartments per month, but he is thinking about asking his roommate to join him so that he can increase the volume of sales. (a) At his current level of activity, Kevin's degree of operating leverage (DOL) is 1.5. How much are his monthly fixed costs? Fixed costs $ per month