The average age of A, B, and C is 17 years old. If D is added, the average age of the four is 19 years old. How old is D?
甲、乙、丙三人的平均年龄17岁，加入丁，四人的平均年龄19岁，那么丁多少岁．
<gadget id="calculator">19 * 4</gadget>
<output>76</output>

<gadget id="calculator">17 * 3</gadget>
<output>51</output>

<gadget id="calculator">76 - 51</gadget>
<output>25</output>

<result>25</result>