The third grade has 310 students, the fourth grade is twice that of the third grade, how many more students are in the fourth grade than the third grade?
三年级有310人，四年级是三年级的2倍，四年级比三年级多多少人？
<gadget id="calculator">310 * 2</gadget>
<output>620</output>

<gadget id="calculator">620 - 310</gadget>
<output>310</output>

<result>310</result>