Guangming Primary School donated 240 books to students in mountainous areas. The fourth grade donated 240 books, the fifth grade donated twice that of the fourth grade, and the sixth grade donated 120 more books than the fifth grade. How many books are donated by each grade on average?
光明小学为山区同学捐书，四年级捐240本，五年级捐的是四年级的2倍，六年级比五年级多捐120本，平均每个年级捐多少本？
<gadget id="calculator">240 * 2</gadget>
<output>480</output>

<gadget id="calculator">480 + 120</gadget>
<output>600</output>

<gadget id="calculator">240 + 480 + 600</gadget>
<output>1_320</output>

<gadget id="calculator">1_320 / 3</gadget>
<output>440</output>

<result>440</result>