water in a cup and a kettle can have the same temperature even though the quantities are different . give reasons

The reason is because both are exposed to a virtually infinite heat sink, due to the virtually infinite mass  and of the surrounding environment, compared to the sizes of either the cup or the kettle such that the equilibrium temperature, reached is the same for both the cup and the kettle as given by the relation;

Due to the large heat sink, T₂ - T₁ ≈ 0 such that the temperature of the kettle and that of the cup will both cool to the temperature of the environment

Explanation:

need to see circuit to get time constant