cumulative frequency table
But sum of frequency Σfi = N = 80
which is equal to last term of c.f. group
∴ 45 + x + y = 80 or x + y = 80 – 45
⇒ x + y = 35 …(i)
Now \(\frac { N }{ 2 }\) = \(\frac { 80 }{ 2 }\) = 40
and median of distribution = 28.5
which lies in class-interval 20 – 30.
Median class = 20 – 30
l = 20, f = 20, c = 5 + x and h = 10
57 = 75 – x or x = 18
Putting value of x in eqn. (i),
18 + y = 35
Thus, y = 17.