f.2 maths graph

我想問
frequency ploygon

佢俾左個classmark你
要我地計番個class interval

classmark
39.5,41.5,43.5,47.5

甘我就諗住甘計
因為class merk等如lower limit 加upper limit,,so
let x be the smaller number in the class interval
x+[x+(41.5-39.5)]/2=39.5
x=40.5

so the class interval應該系40.5-42.5

旦系個答案系41-42

我唔知點解錯-__-help!!