Arbor Day is here, and the senior grades have planted 315 trees, which is three times that of the middle grades. How many trees have been planted in the middle grades?
植树节到了，高年级栽了315棵，是中年级的3倍，中年级栽了多少棵？
<gadget id="calculator">315 / 3</gadget>
<output>105</output>

<result>105</result>