Hope Primary School participated in tree planting. The number of trees planted in the fifth grade was 180, twice that of the third grade. The number of trees planted in the third grade was 45 times less than that in the fourth grade. How many trees were planted in the fourth grade?
希望小学参加植树，五年级种数180棵，是三年级种数的2倍，三年级种的树比四年级种的3倍少45棵，四年级种树多少棵？
<gadget id="calculator">180 / 2</gadget>
<output>90</output>

<gadget id="calculator">90 + 45</gadget>
<output>135</output>

<gadget id="calculator">135 / 3</gadget>
<output>45</output>

<result>45</result>