Calculate the arithmetic mean from the following series by direct method:
Marks
5-15
15-25
25-35
35-45
45-55
No. of students
8
12
15
9
6
"20\\times12=240"
"30\\times15=450"
"40\\times9=360"
"50\\times6=300"
"\\frac{80+240+450+360+300}{8+12+15+9+6}=28.6"
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