Given, power = 90W
time t = 7 min = 420 sec
power = heat supplied / time
or, hjeat supplied Q = 90 * 420
Q = 37800 J
mass m = 1.0 kg
initial temp t1 = 12 °C
final temp t2 = ??
by the formula,
Q= msdt
Q = ms (t2-t1)
T2 = 0.9 + 12
= 12.9 °C