Decane has how many hydrogen atoms?

Decane is an alkane with $10$ carbon atoms. The general chemical formula for straight-chain alkanes is ${\text{C}}_n{\text{H}}_{2n+2}$.Substituting $n=10$:
$$\text{Number of Hydrogen atoms}=2(10)+2=20+2=22$$
Thus, Decane (${\text{C}}_{10}{\text{H}}_{22}$) has $22$ hydrogen atoms.