Q = mc.dT is applicable for physical processes. eg. transfer of heat to water.
Q = mc.dT is only applicable for burning fuel for this case:
1. burn 200g of fuel
2. heat evolved is used to heat water
3. all heat evolved from burning fuel channels to the water. [assumption]
4. water remains in liquid state.
heat evolved for burning 200g of fuel
= heat absorbed by water
= mc.dT where m: mass of water, c: specific heat capacity, dT: change in temperature.