Water flows through an electric shower at a rate of 6kg per minute. Assuming no heat is transferred to the surroundings, what power is required to heat the water by 20K as it flow through the shower?

To find the necessary power, we need to know how much water is flowing every second, and how much energy we need to heat that amount of water.
Amount of water = 6kg per minute = 6/60 kg per second = 0.01 kg per second
energy to heat 0.01kg of water: Q = mCΔT = 0.01 x 4200 x 20= 840J
and so to do that every second the shower needs 840W of power.