Plant 15 trees on each side of a triangle-shaped field, and exactly one tree is planted on each corner. How many trees should be planted on the three sides?
在一块形状是三角形的场地上种数，每条边上都种15棵，每个角上正好有一棵，三条边上一共要种多少棵？
<gadget id="calculator">15 * 3</gadget>
<output>45</output>

<gadget id="calculator">45 - 3</gadget>
<output>42</output>

<result>42</result>