The third grade donated 84 books to children in mountainous areas, and the fourth grade donated twice as many books as the third grade. How many books were donated in total by the two grades?
三年级给山区小朋友捐书84本，四年级捐书是三年级的2倍，两个年级一共捐书多少本？
<gadget id="calculator">84 * 2</gadget>
<output>168</output>

<gadget id="calculator">168 + 84</gadget>
<output>252</output>

<result>252</result>