On a school trip to Nainital, 31 children went boating. How many children went boating but did not take the lake walk?

The Boating circle is the shared part plus the part marked ?: $13+?=31$, so $?=18$. 31 is the whole Boating circle, 13 is the children who did both, and 32 is the Lake-walk circle $19+13$.