A total of 90 trees were planted in the fourth, fifth and sixth grades of a Hope Primary School, among which (2/5) were planted in the sixth grade, (1/3) were planted in the fifth grade, and the rest were planted in the fourth grade. How many trees are planted in fourth grade?
某希望小学四五六年级一共植了90棵树，其中(2/5)是六年级植的，(1/3)是五年级植的，其余的是四年级植的．四年级植树多少棵？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(2/5) + (1/3)</gadget>
<output>11/15 = around 0.733333</output>

<gadget id="calculator">1 - (11/15)</gadget>
<output>4/15 = around 0.266667</output>

<gadget id="calculator">90 * (4/15)</gadget>
<output>24</output>

<result>24</result>