The ratio of 7 to a number is exactly equal to the ratio of 0.8 to 2.4. What is this number?
7与一个数比正好等于0.8与2.4的比，这个数是多少？
<gadget id="calculator">7 * 2.4</gadget>
<output>16.8</output>

<gadget id="calculator">16.8 / 0.8</gadget>
<output>21</output>

<result>21</result>