Four people, A, B, C, and D, are 64 years old this year, of which A is 18 years old, and C is three times as old as D. It is known that when A was 21 years old and B was 17 years old, how old is D now?
甲、乙、丙、丁四人今年共64岁，其中甲18岁，丙的岁数是丁的3倍．已知当甲21岁时，乙17岁，丁现在多少岁．
<gadget id="calculator">21 - 17</gadget>
<output>4</output>

<gadget id="calculator">18 - 4</gadget>
<output>14</output>

<gadget id="calculator">64 - 18 - 14</gadget>
<output>32</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">32 / 4</gadget>
<output>8</output>

<result>8</result>