If you put a cup of hot chocolate at 90c on a table in a room kept at 25c you know it will cool down. How cool will the hot chocolate get?

The final temperature of the hot chocolate will be be greater than the room temperature but less than the initial temperature.

Applying the principles of mixtures, when a hot liquid is  mixed with a cold liquid, the temperature of the resulting mixtures will be greater than the cold liquid temperature but lower than the hot liquid temperature.

The given temperatures;

Let the equilibrium temperature = T

Thus, we can conclude that the final temperature of the hot chocolate will be be greater than the room temperature but less than the initial temperature.

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Explanation:

Assuming we have a two hot liquid like 70°C tea and 30°C water, the tea will cool down but not to 30°C but to an equilibrium temperature say 40°C

But in this case, the temperature of the chocolate will drop almost to the room temperature eventually,

The heat in the chocolate will even out into the room temperature and the room will get slightly warmer until the are both in equilibrium temperature. But you won't notice this little change but if you go out of the room, you might notice that change...

So this little fraction is not always notice, so we will still considered it as the room temperature

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s