Since we have 5 girls and their parents give each other $10, they have in total $50.
Now, let x be the number of pizzas they buy, since each pizza cost $7.25 the total cost for x pizzas is:
this should be less or equal to $50 (otherwise the girls go over budget), then the inequality that represents this situation is:
Now, solving the inequality we get:
Therefore, they can buy a maximum of 6 pizzas without going over budget.