The final answer is 4.246 Molar. For a given volume of the solution, let's say 100mL, the following calculations can be made:
Given:
Molar Mass HBr = 80.904g/mol
Density solution = 1.24 g/mL
27.7%(wt/wt) HBr
Total mass of solution = volume(density) = 100mL (1.24 g/mL) = 124 g solution
Mass HBr in solution = 124g(0.277) = 34.348 g HBr
Moles HBr in solution = 34.348/80.904 = 0.4246 moles HBr
Molarity = moles solute/L solution = 0.4246 moles HBr/0.1 L solution
Molarity = 4.246 M