Question

The percentage by mass of Br in the compound

Answer 1

What you have to do it find the percentage / solve it. Divide the mass by the entire mass. 
239.7 ÷ 266.68 = 0.898 × 100 = 90% Br.

Answer 2

Answer: The correct answer is option(4).

Explanation:

Molar mass of the aluminium bromide, $AlBr_3=266.69 g/mol$

Atomic mass of bromine = 79.90 g/mol

Percentage of Mass of bromine in $AlBr_3$:

$\% Br\text{ by mass}=\frac{\text{Atomic mass of Br}\times \text{Number of Br atoms}\times 100}{\text{Molar mass of }AlBr_3}$

$\% Br\text{ by mass}=\frac{79.90 g/mol\times 3\times 100}{266.69 g/mol}=89.97\%\approx 90\%$

Hence, the correct answer is option(4).