Gus has a lunch food budget of $150 per month. He spends an average of $7 per day on lunch. Which of the following equations represents how much money is left in his lunch budget after x days of buying lunch?

If your asking for the equation to solve the problem i would have to say take 7 times x days then subtract that solution from 150 to get your final answer but just so you know at 7 a day and 150 a month he can only eat 21 days.