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ICSE Class X Math 2013 Solved Paper

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Question : 37
Total: 46
Find the mean of the following distribution by step deviation method:
 Class Interval  20-30  30-40  40-50  50-60  60-70  70-80
 Frequency  10  6  8  12  5  9
Solution:  
 C.I.  Frequency (fi)  Mid-value(x)  di=‌
x−a
h
  fidi
 20-30  10  25  -2  -20
 30-40  6  35  -1  -6
 40-50  8  45  0  0
 50-60  12  55  1  12
 60-70  5  65  2  10
 70-80  9  75  3  27
   Σfi=50      Σfidi=50

Here, a=45 and h=10
∴‌‌‌ Mean ‌=a+‌
Σfidi
Σfi
×h
‌=45+‌
23
50
×10
‌=45+4.6=49.6
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