The average of the three numbers is 24.5, the number A is twice the number B, and the number C is half of the number B, what is the number B?
三个数的平均数是24.5，甲数是乙数的2倍，丙数是乙数的一半，求乙数的多少？
<gadget id="calculator">24.5 * 3</gadget>
<output>73.5</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 + 2 + (1/2)</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">73.5 / (7/2)</gadget>
<output>21</output>

<result>21</result>