Given:
Mass of sample A and B, mA=mB=400 g=0.4 kg
Temperature of sample A, TA=20∘C
Temperature of sample B, TB=65∘C
Specific heat of water, Sw=4186 J/kg K
The heat lost by sample A will be equal to heat gained by sample B.
Let the temperature of the mixture be Tm.
Heat lost = Heat gained
∴ mASw(TB−Tm)=mBSw(Tm−TA)
⇒0.4×(65−Tm)=0.4×(Tm−20)
⇒2Tm=85
⇒Tm=42.5∘C