There are 4 rows of trees in the campus, each row has 13 trees, and some trees are planted in spring, so that there are 69 trees in the campus in total. How many trees were planted in spring?
校园里有4行树，每行13棵，春天又种了一些树，这样校园里一共有69棵树．春天种了多少棵树？
<gadget id="calculator">13 * 4</gadget>
<output>52</output>

<gadget id="calculator">69 - 52</gadget>
<output>17</output>

<result>17</result>