Surveying 20 students

showed

Mark Number of Student 31-39 2 40-48 3 49-57 5 58-66 4 67-75 3 76-84 2 85-93 1

Find Quartile Deviation and Mode

Help Plz!! I got the Mode=50.16

But I'm confuse with the Quartile Deviation on the First Quartile they used c.f.= 2 or 5 @_@