Question

100g of lukewarm coffee at 40'c left in a paper cup. when you pour 50g of boiling water into cup , the temputure of the resulting coffee -like mixture will be now

Answer 1

heat is transferred from the water is opposite to the heat transferred to the coffee.
\[mC(T_{final}-T_{initial})|_{coffee}=-mC(T_{final}-T_{initial})|_{water}\]
heat capacity for coffee and water will essentially be the same so you're left with
\[(100~g)(T-40°C)=-(50~g)(T-100°C)\]