Finding equilibrium temperature when two things are mixed
In this type of question, you should bring all the matter into one state and calculate net heat.
In this solution, we will consider heat released as positive heat, absorbed as negative, and the state to which we will bring all the matter be water at 0 °C.

Here we were left with 20 g of water at 0 °C and zero heat if heat was some positive quantity we would use the heat to raise 20 g water to a specific temperature.
By equation $20\times T\times1=\text{heat left}$
$T$ is temperature to which 20 g water is raised
If heat was negative quantity, we would convert part of water at 0 °C to ice at 0 °C by equation
$M\times80=\text{heat left}$
$M$ mass of water which is converted to ice