On the "June 1st" festival, Mr. Wang led 40 students from Class 4 and 1 to visit Dongpo Chibi, and bought tickets for a total of 840 yuan. It is known that the price of an adult ticket is equal to the price of two student tickets, how much is a student ticket?
"六一"节，王老师带领四1班的40名学生游东坡赤壁，买门票共用了840元．已知一张成人票的价钱与两张学生票的价钱相等，一张学生票多少钱？
<gadget id="calculator">40 + 2</gadget>
<output>42</output>

<gadget id="calculator">840 / 42</gadget>
<output>20</output>

<result>20</result>