How many of the following methyl halides have higher density than ${\mathrm{CH}}_3\mathrm{I}$ ?

For a given alkyl group, alkyl iodide has the highest density; therefore the halide must be iodine. Since, the density decreases as the size of alkyl group increases; the alkyl must be methyl.

Therefore, methyl iodide has the highest density among the other methyl halides:

${\mathrm{CH}}_3\mathrm{Cl}, {\mathrm{CH}}_3\mathrm{F}, \mathrm{and} {\mathrm{CH}}_3\mathrm{Br}$