Question

in a school there are 600 students 400 speak Telugu 300 speak Hindi how many speak both telugu and hindi

Answer 1

if there r 600 students 
400 speak telugu and 300 speaks hindi 
100 know both

Answer 2

n(T union H) = n(T) + n(H) - n (T intersection H)
600 = 400 + 300 - n (T intersection H)
600-700 = - n (T intersection H)
100 = n (T intersection H)