The sum of the current ages of A, B, C, and D is 64 years old. When A is 21 years old, B is 17 years old; when A is 18 years old, C is three times as old as D. How old is Ding now?
甲、乙、丙、丁现在的年龄和是64岁．甲21岁时，乙17岁；甲18岁时，丙的年龄是丁的3倍．丁现在的年龄是几岁？
<gadget id="calculator">21 - 18</gadget>
<output>3</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">64 - 21 - 17 - 6</gadget>
<output>20</output>

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

<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<gadget id="calculator">5 + 3</gadget>
<output>8</output>

<result>8</result>