The school planted 15 poplars, and the number of willows planted was three times that of poplars. How many willows were planted?
学校种植杨树15棵，种植的柳树是杨树的3倍，种柳树多少棵？
<gadget id="calculator">15 * 3</gadget>
<output>45</output>

<result>45</result>