A ask about a set of data that can vary

Aschool puts on a play. the play costs $1,200 in expenses. the students charge $4.00 for tickets. there will be one performance of the play in an auditorium that seats 500 people. what is the domain of the function that shows the profit as a function of the number of tickets sold?  

You are at a restaurant and owe $56.20 for your meal. you want to leave an 18% tip. approximately how much is your tip?

What is the value of x in the equation7x+2y=48 when y=3