Xiao Ming's father bought 20 bottles of juice for him. The store stipulates that every 7 empty bottles can be exchanged for 3 bottles of juice. Xiao Ming wants to exchange empty bottles for juice as much as possible. How many bottles can he drink in total?
小明的爸爸为他买了20瓶果汁，商店规定每7个空瓶可换回3瓶果汁，小明想尽量用空瓶换果汁喝，这样他一共可喝到多少瓶？
<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">20 + 9 + 3 + 3</gadget>
<output>35</output>

<result>35</result>