What is the average height of the class?
I. Average height of the class decreases by 1 cm i we exclude the tallest person of class whose height is 56 cm.
II. Average height of the class increase by 1 cm if we exclude the shortest person o the class whose height is 42 cm.

Let x be the average height of the class and n be the number of students in the class.
Consider statements I alone
xn - 56 = (x + 1)(x -1)
? x + n = 57 .............(i)
Hence, the value of x cannot be found. So, I alone is not sufficient.
Consider statement I alone:
xn - 42 = (x + 1)(n - 1)
? x - n = 41 .............(ii)
Hence, the value of x cannot be found. So, II alone is not sufficient.
Both the statements together are suficient as the value of x can be found by solving (i) and (ii)