1 g of non-volatile non-electrolyte solute is dissolved in 100 g of two different solvents A and B whose ebullioscopic constants are in the ratio of 1:5. The ratio of the elevation in their boiling points, ΔTb(A):ΔTb(B), is:

We know, ΔTb = Kb × m
where ΔTb is the elevation in boiling point, Kb is the ebullioscopic constant, and m is the molality of the solution.
Given that 1 g of solute is dissolved in 100 g of solvent A and 100 g of solvent B, the molality of the solute in both solvents will be the same.
Let the molality be m.
For solvent A, ΔTb(A) = Kb(A) × m
For solvent B, ΔTb(B) = Kb(B) × m
The ratio of ebullioscopic constants is given as Kb(A):Kb(B) = 1:5
Therefore, Kb(A) = x and Kb(B) = 5x for some constant x.
The ratio of elevation in boiling points is:
ΔTb(A)/ΔTb(B) = (Kb(A) × m) / (Kb(B) × m) = Kb(A) / Kb(B) = x / 5x = 1/5
Therefore, the ratio of the elevation in their boiling points is 1:5.