Answer : The unknown mass of water is, 200.3 grams.
Explanation :
In this problem we assumed that heat given by the hot body is equal to the heat taken by the cold body.


As,
= specific heat water
So,

where,
= mass of water = 150 g
= mass of unknown water = ?
= final temperature of mixture =

= initial temperature of water =

= initial temperature of unknown water =

Now put all the given values in the above formula, we get


Therefore, the unknown mass of water is, 200.3 grams.