Given three numbers 12, 16, and 9, if one more number is added so that it can form a proportional formula with the known three numbers, how much should this number be?
已知三个数12、16、9，如果再添上一个数，使之能与已知三个数组成比例式，这个数应该是多少？
<gadget id="calculator">12 / 16</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">(3/4) * 9</gadget>
<output>27/4 = around 6.75</output>

<result>27/4 = around 6.75</result>