Henry plans to buy Pepsee for people to drink at his birthday party. The grocery store has a sale
 in which the first liter of Pepsee costs $3, and every liter that is bought after that costs $1.50.
 Henry has $9 to spend.
 What is the maximum number of liters Henry can buy? 4 liters
 4.67 liters
 5 liters
 6 liters