P = 1000
V = 220v
R = V2/P = 48400/1000 = 48.4Ω
Mass of water = 1/100 x 1000 = 10kg
Heat required to raise the temp. of given amount of water = msΔt = 10 × 4200 × 25 = 1050000
Now heat liberated is only 60%. So V2/R x T x 60% = 1050000
=> (220)2/48.4 x 60/100 x T = 1050000 => T = 10500/6 x 1/60nub = 29.16min