How do I write an inequality for the following word problem?

Question

My teacher requires an inequality, and I have an answer but no idea how to write an inequality for it. 

" The rule of thumb in filmmaking is that you must shoot at least three minutes of film for every minute in a movie's "final cut." A 30 minute roll of film cost $250. How much will film cost to make a 90 minute movie?

Answer 1

 90 minutes x $250 / 30 mintes = 

 

3 x $250 = $750