On Saturday (Class 6.2), 31 students went to the park to row boats. There are two kinds of boats in the park. The large boat can take 9 people, and the rent is 20 yuan per hour. The small boat can take 4 people, and the rent is 9 yuan per hour. How much is the minimum cost for rowing for 1 hour?
星期六(6.2班)31名同学到公园去划船．公园里有两种船，大船每条乘9人，每小时租金20元，小船每条乘4人，每小时租金9元，如果划1小时，最少要花多少元？
<gadget id="calculator">20 * 3</gadget>
<output>60</output>

<gadget id="calculator">60 + 9</gadget>
<output>69</output>

<result>69</result>