Cumulative frequency problems

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This is very simple maths but I just can't get my head around it.

 

1) My first question (KEEP IN MIND THAT > SHOULD ACTUALLY INCLUDE EQUALS TO ALSO - I JUST COULDN'T PUT THE UNDERSCORE)

 

I have a frequency table as follows:

 

1Q MIDPOINT FREQUENCY FX CUMULATIVE FREQUENCY

 

80 > 85 82.5 1 1

85 > 90 87.5 - -

90 > 95 92.5 7 8

95 > 100 97.5 4 12

 

THE QUESTION: The 85 > 90 CLASS INTERVAL HAS NO FREQUENCY.

How would I deal with this when I draw the cumulative frequency curve (graph). Would I start at 1 and go down to 0 and then draw the curve up to 8 and then to 12. WOULD I HAVE TO GO DOWN TO 0.

 

My final QUESTION concerns the midpoints. ARE THEY CORRECT? Since the cumulative frequency graph is plotted at the class interval boundary (e.g. 85.5, 90.5, 95.5 etc) - will the midpoint change to 82.75, 90.75. instead of82.5 etc)

 

 

I took a long time writing this post so please answer both questions.

Thanks in advance.