The students have to plant 84 trees, and they have already planted 3 rows with 15 trees in each row. How many trees are left to be planted?
同学们要栽84棵树，已经栽了3行，每行15棵，还剩多少棵没栽？
<gadget id="calculator">3 * 15</gadget>
<output>45</output>

<gadget id="calculator">84 - 45</gadget>
<output>39</output>

<result>39</result>