A total of 110 trees were planted in the fourth, fifth and sixth grades. The number of trees planted in the sixth grade was 3 times that of the fourth grade and 1 less than that of the fourth grade. The number of trees planted in the fifth grade was 3 times more than that of the fourth grade. Then, how many trees were planted in the fourth grade? ．
四、五、六年级共植树110棵，六年级植的棵数是四年级的3倍少1棵，五年级植的棵数是四年级的2倍多3棵，那么，四年级植树多少棵．
<gadget id="calculator">1 - 3</gadget>
<output>-2</output>

<gadget id="calculator">110 + (-2)</gadget>
<output>108</output>

<gadget id="calculator">3 + 2 + 1</gadget>
<output>6</output>

<gadget id="calculator">108 / 6</gadget>
<output>18</output>

<result>18</result>