Explanation:
Let thermal capacity of the vessel be C' J K-1
Heat energy given by hot water = 40 x 4.2 x (60 - 30) = 5040 J
Heat energy taken by cold water = 50 x 4.2 x (30 - 20) = 2100 J
Heat energy taken by vessel = C' x (30 - 20) = 10 C' J
If there is no loss of heat energy,
Heat energy given by hot water = Heat energy taken by cold water and vessel
or 5040 = 2100 + 10 C'
or 10 C' = 2940
or C'= 294 J K-1
Thus, thermal capacity of vessel = 294 J K-1.