Xiao Ming and his father went to the zoo to play on Sunday. Student fares are half of adult fares. Xiao Ming paid 20 yuan, and there is still a difference of 2.5 yuan. How much is the student ticket?
小明和爸爸星期天到动物园玩．学生票价是成人票价的一半．小明付了20元，还差2.5元，学生票是多少元？
<gadget id="calculator">20 + 2.5</gadget>
<output>22.5</output>

<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">22.5 / 3</gadget>
<output>7.5</output>

<result>7.5</result>