I need some math help really easy!?

Billy's Burgers surveyed customers about the condiments they like on their burger. Thirteen customers liked mustard only, 8 customers liked mayo only, and 17 customers liked at least ketchup on their burger. If seven customers liked their burgers with none of these condiments, and 50 total customers were surveyed, how many customers liked mayo and mustard without ketchup on their burger?