The average of the three numbers A, B, and C is 15. A: B: C = 2: 3: 4. What is the number of C?
甲、乙、丙三个数的平均数是15，甲：乙：丙=2：3：4，丙数是多少．
<gadget id="calculator">15 * 3</gadget>
<output>45</output>

<gadget id="calculator">2 + 3 + 4</gadget>
<output>9</output>

<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">45 * (4/9)</gadget>
<output>20</output>

<result>20</result>