If the total expenses are $90/day, your the business is profitable if the amount you sold minus your expenses is greater than 0.
For example, if you only sold $80 worth of food that day, the food does not cover the cost of the expenses. However, if you sold $100 worth of food that day, $90 of that amount goes to your expenses & you get to keep the other $10.
So, the business is profitable if:
s - 90 > 0, where s is the amount sold in dollars that day.
So, your business is profitable if you sell more than $90 worth of goods.
Hope this helps!