Four people, A, B, C, and D, produce parts. They produce an average of 13 parts each; A, B, and C each produce an average of 12 parts; C and D produce an average of 15 parts each. Find C How many parts are produced.
甲、乙、丙、丁四个人生产零件，他们平均每人生产零件13个，甲、乙、丙三人平均每人生产零件12个，丙、丁两人平均每人生产15个，求丙生产零件多少个．
<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<gadget id="calculator">13 * 4</gadget>
<output>52</output>

<gadget id="calculator">12 * 3</gadget>
<output>36</output>

<gadget id="calculator">52 - 36</gadget>
<output>16</output>

<gadget id="calculator">30 - 16</gadget>
<output>14</output>

<result>14</result>