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? Think carefully first, then make a decision:
There are 10 / 100 * 4000 = 400 spruces in the forest. There are 13 / 100 * 4000 = 520 pines in the forest. There are as many oaks as pines and spruces together, so there are 400 + 520 = 920 oaks. Therefore there are 4000 - 920 - 520 - 400 = 2160 birches. So, the answer is 2160.