The students wanted to plant some trees. On the first day they planted 10 trees, on the second day they planted the remaining half, on the third day they planted another 9 trees, and on the fourth day they planted 6 trees and they just finished planting. How many trees are there?
同学们要植一些树，第一天他们植了10棵，第二天他们植了剩下的一半，第三天又植了9棵，第四天植了6棵就正好植完，这些树一共有多少棵．
<gadget id="calculator">6 + 9</gadget>
<output>15</output>

<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<gadget id="calculator">30 + 10</gadget>
<output>40</output>

<result>40</result>