The percentage of halogen by mass tells us how much of a compound is made of halogen…

Step 1. Dichloromethane. $M = 12.0 + 2 \times 1.0 + 2 \times 35.5 = 85.0\ \text{g/mol}$. Chlorine: $\dfrac{71.0}{85.0} \times 100 = 83.5\ \%$.

Step 2. Chloroform. $M = 12.0 + 1.0 + 106.5 = 119.5\ \text{g/mol}$. Chlorine: $\dfrac{106.5}{119.5} \times 100 = 89.1\ \%$.

Step 3. Tetrachloromethane. $M = 12.0 + 142.0 = 154.0\ \text{g/mol}$. Chlorine: $\dfrac{142.0}{154.0} \times 100 = 92.2\ \%$.

Step 4. Iodoform. $M = 12.0 + 1.0 + 381.0 = 394.0\ \text{g/mol}$. Iodine: $\dfrac{381.0}{394.0} \times 100 = 96.7\ \%$.