In a certain forest, there are 4 species of trees: oak, pine, spruce, and birch. There is a total of 4000 trees in the forest. Spruces make up 10% of all the trees, and pines 13%. There are as many oaks as spruces and pines put together. How many birches are there in the forest?
There are 10/100 * 4000 = 
<gadget id="calculator">10/100*4000</gadget>
<output>400</output>
400 spruces in the forest.
There are 13/100 * 4000 = 
<gadget id="calculator">13/100*4000</gadget>
<output>520</output>
520 pines in the forest.
There are as many oaks as pines and spruces together, so there are 400 + 520 = 
<gadget id="calculator">400+520</gadget>
<output>920</output>
920 oaks.
Therefore there are 4000 - 920 - 520 - 400 = 
<gadget id="calculator">4000-920-520-400</gadget>
<output>2_160</output>
2160 birches.

<result>2_160</result>