A cup of boiling water (100 degrees C) (0.2kg) transfers 20 000J of heat to its surrounding in 1 hour. What temperature is the water in the kettle after 1 hour?

To answer this we need to use the equation Q = MC(T1-T2)

We want to know what T2 is (ie the temperature after one hour).

So rearrange the equation to get T2 = T1 - Q/MC

No substitute the values given: T2 = 100 - 20000/(0.2 x 4200) = 76.2 ⁰C