To calculate the percentage of bromine in the compound X, we first need to determine the moles of AgBr formed and then the moles of bromine present in the compound. Finally, we can find the percentage of bromine by weight.

1. Calculate the moles of AgBr formed:

Moles of AgBr = mass / molar mass = 0.376 g / 188 g/mol = 0.002 mol

Since 1 mol of AgBr contains 1 mol of Br, the moles of Br in the compound X are also 0.002 mol.

2. Calculate the mass of bromine in the compound:

Mass of Br = moles of Br × molar mass of Br = 0.002 mol × 80 g/mol = 0.16 g

3. Calculate the percentage of bromine in the compound X:

Percentage of Br = (mass of Br / mass of compound X) × 100 = (0.16 g / 0.400 g) × 100 = 40%

So, the percentage of bromine in the compound X is 40%.