The students went to plant trees. The first grade planted 47 trees, the second grade planted 54 trees, and the third grade planted 17 fewer trees than the first and second grades. How many trees did the third grade plant?
同学们去植树，一年级栽了47棵，二年级栽了54棵，三年级栽的比一、二年级栽的总棵数少17棵．三年级栽了多少棵树？
<gadget id="calculator">54 - 17</gadget>
<output>37</output>

<gadget id="calculator">47 + 37</gadget>
<output>84</output>

<result>84</result>