Students from Xingfu Elementary School planted trees on Arbor Day. The middle grades planted 68 trees, twice as many as the lower grades, and the upper grades planted 9 fewer trees than the lower grades. How many trees are planted in the senior grades of Xingfu Primary School?
幸福小学的学生植树节植树．中年级植树68棵，是低年级的2倍，高年级植的比低年级的2倍少9棵．幸福小学高年级植树多少棵？
<gadget id="calculator">68 / 2</gadget>
<output>34</output>

<gadget id="calculator">34 * 2</gadget>
<output>68</output>

<gadget id="calculator">68 - 9</gadget>
<output>59</output>

<result>59</result>