Star Cinema has 15 seats in each row, and a family of 3 can go to the movies. How many different ways are there to buy a seat ticket for the 20th row?
星空电影院每排有15个座位，一个3口之家去看电影．要买第20排的连座票，一共有多少种不同的拿法．
<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<gadget id="calculator">12 + 1</gadget>
<output>13</output>

<result>13</result>