The gymnasium has a total of 840 basketball and volleyball seats. There are 20 boxes of basketballs, 24 in each box; 18 boxes of volleyballs, how many boxes are there on average?
体育馆有篮球和排球共840个．篮球有20箱，每箱24个；排球有18箱，平均每箱多少个？
<gadget id="calculator">24 * 20</gadget>
<output>480</output>

<gadget id="calculator">840 - 480</gadget>
<output>360</output>

<gadget id="calculator">360 / 18</gadget>
<output>20</output>

<result>20</result>