Let's use the concept of conservation of energy.
Q for granite = Q for water
(mass of granite)(Cs of granite)(ΔT) = (mass of water)(Cs of water)(ΔT)
where ΔT is the difference of the initial temperature, T₀, and the equilibrium temperature T. T would be between 82°C and 22°C. Make sure that ΔT is positive.
T₀ for granite = 82°C
T₀ for water = 22°C
Substituting the values and solving for T,
(12.5)(0.79)(82 - T) = (25)(4.18)(T - 22)
T = 27.18°C