If the average age of four people is 30 years old, and none of the four people is younger than 21 years old, what is the age of the oldest one?
如果四个人的平均年龄是30岁，且在四个人中没有小于21岁的，那么年龄最大的这个是多少岁．
<gadget id="calculator">30 * 4</gadget>
<output>120</output>

<gadget id="calculator">21 * 3</gadget>
<output>63</output>

<gadget id="calculator">120 - 63</gadget>
<output>57</output>

<result>57</result>