In the ratio, the two outer terms are reciprocals of each other, one of the inner terms is (4/9), and the other inner term is how much.
在比例中，两个外项互为倒数，其中一个内项是(4/9)，另一个内项是多少．
<gadget id="calculator">9 / 4</gadget>
<output>9/4 = around 2.25</output>

<result>9/4 = around 2.25</result>