Answer
12 Hydrogen atoms
Work Step by Step
Since we know that the molecule has 3 double bonds and 1 ring, so we know it has 4 elements of unsaturation.
Applying the rule $\frac{1}{2}\times(2C+2-H)=$ elements of unsaturation.
so $\frac{1}{2}\times(18+2-H)=4$
then, H = 12.
therefore we have 12 Hydrogens in the molecule with formula $C_{9}H_{12}$.
Another simple way is by knowing the Saturated Formula which is $C_{n}H_{2n+n}$ and our molecule is $C_{9}H_{20}$
and by knowing that with each element of unsaturation, we subtract 2 hydrogens then, 2x4=8 hydrogen and 20-8=12 Hydrogen then $C_{9}H_{12}$.
