The two inner terms of the ratio are 2 and 5, and the two outer terms are x and 2.5. What is x?
比例的两个内项分别是2和5，两个外项分别是x和2.5．x是多少？
<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">10 / 2.5</gadget>
<output>4</output>

<result>4</result>