Let the final temperature of the mixture be T.
Then 100×80+100(T−0)×1/2100×80+100(T−0)×1/2
(as specific heat of ice is 0.5cal/g/℃ and specific heat of water is 1cal/g∘C)
=100×1×(100−T)
Solving, we get T=13.33℃