How much water can be heated from 15 to 100C using 1 kWh of energy if the efficiency of the kettle is 50%

Efficiency of the kettle: η = A / Q, where η is the efficiency of the kettle (η = 50%), A is the useful work of the kettle, Q is the amount of heat (Q = 1 kWh = 1000 W * 3600 s = 3.6 MJ).
Let us express the useful work of a teapot:
A = Q * η = 3.6 * 0.5 = 1.8 MJ.
We express the mass of heated water from the formula:
A = C * m * ∆t, where C is the specific heat capacity of water (C = 4187 J / (kg * K)), m is the mass of heated water (kg), ∆t is the temperature change (∆t = t1-t2 = 100 – 15 = 85K).
m = A / (C * ∆t) = (1.8 * 10 ^ 6) / (4187 * 85) = 5.06 kg.
Answer. 5.06 kg of water can be heated.