Counting alkanes

How many isotopes are there of $C_4H_{10}, C_5H_{12}$ and $C_6H_{14}?$

Counting Leaves

Prove that if a tree $T$ has a vertex $v$ of degree 4 and a vertex $w$ of degree 3 then $T$ has at least five leaves.

Counting Atoms

Suppose we know that a molecue is composed of Hydrogen, Nitrogen and Carbon, is a tree, and has 10 hydrogen atoms. What possible chemical formulas could it have? As a bonus, find all isomers for each of those chemical formulas.