The students participated in the summer camp. There are 21 students in the lower grades participating in the summer camp, and the number of senior students participating in the summer camp is three times that of the lower grades.
同学们参加暑假夏令营，低年级参加夏令营的有21人，高年级参加夏令营的人数是低年级的3倍，高年级参加夏令营的人数比低年级多多少人？
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<result>42</result>