The second class of the fourth grade of a certain school participated in the donation activity to the disaster area. The 45 people in the first class donated a total of 315 yuan; the 42 people in the second class donated an average of 8 yuan per person. How much is the donation of class one less than that of class two?
某学校四年级二个班参加向灾区捐款活动．一班45人共捐款315元；二班42人，平均每人捐款8元．一班比二班少捐款多少元？
<gadget id="calculator">8 * 42</gadget>
<output>336</output>

<gadget id="calculator">336 - 315</gadget>
<output>21</output>

<result>21</result>