Find the third quartile class and frequency of the class from the given graph. How many students are there above the class? 
My attempt :
Here, $N=120$.
Third quartile lies in $\frac {3N}{4}$item.
$$=90 item$$
So, $Q_3 class = 20-30$.
I have previously asked this question at Graph-Third Quartle
But the actual answer given in my book is $30$ and the answer given by occasional user there is $20$. How is this difference? Which one is correct?
Upper quartile = $\frac{3N}{4}$
N = 120.
Upper quartile = 90th term
Through 90 on the y-axis draw a line parallel to y-axis meeting the curve at P.
From M draw a perpendicular on x-axis which meets it at Q.
You get value 25 on x-axis.
And class is 20-30.