From 8:00 to 12:00, the staff purified 840 square meters of seawater. How many square meters are purified per hour on average?
从8时到12时，工作人员给840平方米的海水进行了净化，平均每小时净化多少平方米？
<gadget id="calculator">12 - 8</gadget>
<output>4</output>

<gadget id="calculator">840 / 4</gadget>
<output>210</output>

<result>210</result>