Question

one liter of water at 20 Celsius mixed with one liter water at 30 Celsius . the temperature of mixture

Answer 1

The colder water will warm up (heat energy "flows" into it). The warmer water will cool down (heat energy "flows" out of it).
So 1 litre of water (20⁰C) mixed with 1 litre of water (30⁰C)
20--------------x---------------30
the answer will be 25⁰C.

Answer 2

when water A at 20°C is added to water B at 30°C then heat from B flows to A.the final tenperature of the mixture would be (20+30)/2 = 25°C.