Make an equation to solve this problem. The cost of a gallon of orange juice is $3.50, so we can show this using the expression 3.50o, where o represents the number of gallons of orange juice that someone buys.
We can put the expression 3.50o into an equation, and make the expression equal to 15 because that is the amount of money you supposedly have.
3.50o = 15, divide both sides of the equation by 3.50.
o = 4.28571428571, but you cannot have a fraction or decimal of a gallon of orange juice, so we would always round these kinds of problems down to the nearest whole number.
4.28571428571 rounds down to 4 gallons of orange juice, so the answer is 4 gallons of orange juice is the maximum amount you can buy with $15.