m₁ = 2 kg
t₁ = 50 C
cw₁ = 900 J/kg * C
m₂ = 1,5 kg
t₂ = 18 C
cw₂ = 4200 J/kg * C
tk = ?
Q₁ = Q₂
m₁ * cw₁ * (t₁ - tk ) = m₂ * cw₂ * ( tk - t₂)
2 kg * 900 J/kg * C * ( 50 C - tk ) = 1,5 kg * 4200 J/kg * C * ( tk - 18 C)
1800 J/C ( 50 C - tk ) = 6300 J/C * ( tk - 18 C)
90000 J - 1800 J/C * tk = 6300 J/C * tk - 113400 J
90000 J + 113400 J = 6300 J/C * tk + 1800 J/C * tk
203400 J = 8100 J/C * tk
tk = 25,1 C