On Arbor Day, students planted trees together. The third grade planted 45 trees, the fifth grade planted twice as many trees as the third grade, and the fourth grade planted 12 fewer trees than the fifth grade. How many trees did the fourth grade plant?
植树节同学们一起去植树，三年级种了45棵，五年种的棵树是三年级的两倍，四年级种的棵树比五年级少12棵，求四年级植树多少棵？
<gadget id="calculator">45 * 2</gadget>
<output>90</output>

<gadget id="calculator">90 - 12</gadget>
<output>78</output>

<result>78</result>