The fifth graders planted 30 trees this year, which is 15 fewer than last year's three times. How many trees were planted last year?
五年级同学今年植树30棵，比去年植树的3倍少15棵，去年植树多少棵？
<gadget id="calculator">30 + 15</gadget>
<output>45</output>

<gadget id="calculator">45 / 3</gadget>
<output>15</output>

<result>15</result>