One litre of water at 40 degree celsius mixed with one litre of water at 60 degree celsius. What will be the temperature of the mixture ?

Dear student 
Using heat gained = heat lost
  let T  be temperature of mixture the change of temp will continues till  mix water acquires a constant temp
          msΔt₁ = msΔt₂       (here Δt₁ and  Δt₂​   are change in temp occurred during mixing of water at different temp)
                                          (m= mass of water , s = specific heat of water)
               Δt₁ =  Δt₂
                   T - 30 = 30 -T
                  2T =60
                    T =30⁰
 hence temp of mixture is 30 degree 

Regards