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$$10$$ trees are planted at equal distance along a river. The distance between the $$1^{st}$$ and the $$3^{rd}$$ tree is $$20$$ metres. What is the distance between the $$7^{th}$$ and $$9^{th}$$ tree? 
Overseas Competition->Knowledge Point->Word Problem Modules->Interval Problems
They are having the same number of intervals. 