# How many structural isomers of C_3H_9N exist?

If you start with a three-carbon chain, that is 3 carbons and $3 + 2 + 2 = 7$ hydrogens. Then, adding an ${\text{NH}}_{2}$ gives one nitrogen and the 8th and 9th hydrogens. Hence, one $\text{C"_3"H"_9"N}$ structural isomer is the primary amine propylamine.
You can only move a methyl group onto one new carbon before you run into the nitrogen, giving one more structural isomer (isopropylamine). $3 + 3 + 1 + 2 = 9$ hydrogens still, so still $\text{C"_3"H"_9"N}$.
Nitrogen can only accommodate two more methyl groups in place of its hydrogens before you run out of methyl groups to move, giving two more structural isomers (ethylmethylamine and trimethylamine). The former still has $3 + 2 + 1 + 3 = 9$ hydrogens, and the latter still has $3 + 3 + 3 = 9$ hydrogens, so they are both still $\text{C"_3"H"_9"N}$.