sir this is 3 mark question
Q.25. A survey was conducted of the T.V programmes watched by 120 students of a school hostel revealed that 70 students watched 'Discovery channel' and 56 students watched 'sports channel' where as 24 watched both the programmes. Find the number of students who did not watch T.V on that day.

Here, 
given: n(U) = 120, n(Discovery) = 70 , n(Sports) = 56
To find : n(SUD)'
Solution: -
first we have to find n(SUD)
we can find this by the formula;-
n(SUD) = n(D) + n(S) - n(S intersection D)


By putting the given values
n(SUD) = 70 + 56 - 24
hence, n(SUD) = 102
now 102 is the no. of people t.v that day.

To find the no. of people who did not watch t.v that day:-
n(U) - n(SUD) = n(SUD)'

So, 120 - 102 = 18
hence, 18 is the number of people who did not watch t.v on that day.