It takes 4200 Joules to heat up 1 kg of water by 1 C^o. If you have a kettle with a power of 2500 Watts, how long will it takes to boil 0.8 kg of water from an initial temperature of 25 C^o to the nearest second??

Energy required calculated using the equation for specific heat capacity: mcdT = dE where m is mass, c is specific heat capacity and dT is the change in temperature.
Time taken is calculated using the relation between energy, power and time: E / P = t. Substituting in from the previous equation: m * c * dT / P = t. Finally substituting in the numbers: 0.8 * 4200 * (100-25) / 2500 = 100.8 seconds.
Answer: 101 seconds.