The number of pine trees is 15, the number of poplars is three times that of pine trees, and the number of willows is 6 fewer than the total number of pine trees and poplars. How many willow trees are there?
松树有15棵，杨树的棵数是松树的3倍，柳树的棵数比松树和杨树的总棵数少6棵．柳树有多少棵？
<gadget id="calculator">15 * 3</gadget>
<output>45</output>

<gadget id="calculator">15 - 6</gadget>
<output>9</output>

<gadget id="calculator">45 + 9</gadget>
<output>54</output>

<result>54</result>