Since the mole ratio is 1 to 1 for this reaction, you can just use M1V1 = M2V2 to solve, given that 1 refers to HBr and 2 refers to LiOH.
M1 = ?
V1 = 0.40 L
M2 = 0.50 M
V2 = 0.20 L
M1(0.40) = (0.50)(0.20)
Solve for M1 —> M1 = (0.50)(0.20)/0.40 = 0.25 M HBr
