There are 4 rows of trees in the campus, and each row has 15 trees. Some trees were planted this spring, and now there are 105 trees in total. How many trees were planted in spring?
校园里有4行树，每行15棵，今年春季又种了一些树，现在共有105棵树．春季种了多少棵树？
<gadget id="calculator">4 * 15</gadget>
<output>60</output>

<gadget id="calculator">105 - 60</gadget>
<output>45</output>

<result>45</result>