When two bodies of different temperatures are brought in thermal contact then reception and donation of heat take place. The body at higher temperature leaves heat whereas the body at lower temperature receives heat. This give and take will continue until they reach at the thermal equilibrium.

If no heat is lost during donation and reception of heat, then the amount of heat given up by the body at higher temperature will be equal to the heat gained by the body at lower temperature.

So, Heat lost = Heat gained