They are having the same number of intervals. 
$$10$$ 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? 
Options:
[a] $$10$$ 
[b] $$20$$ 
[c] $$50$$ 
[d] $$70$$ 
[e] $$90$$ 