The propagation speed of light in the air is 300,000 kilometers per second. When it propagates in water, the speed will decrease (1/4).
光在空气中的传播速度是每秒30万千米，在水中传播时，速度会降低(1/4)，光在水中传播时每秒的速度降低了多少万千米？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">30 * (1/4)</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>