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Thirty women were examined in a hospital by a doctor and the number of heart beats per minute were recorded and summarised as follows. Find the mean heart beats per minute for these women, choosing a suitable method.
Number of heart beats per minute
65−68
68−71
71−74
74−77
77−80
80−83
83−86
Number of women
2
4
3
8
7
4
2
MEDIUM
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ANSWER
Class interval Mid value (x
i
) (f
i
) u
i
=
3
x
i
−75.5
f
i
u
i
65−68 66.5 2 −3 −6
68−71 69.5 4 −2 −8
71−74 72.5 3 −1 −3
74−77 75.5 8 0 0
77−80 78.5 7 1 7
80−83 81.5 4 2 8
83−86 84.5 2 3 6
Total 30 4
Lets take assumed mean, A as 75.5
Class interval =3
∴u
i
=
h
x
i
−A
=
3
x
i
−75.5
x
ˉ
=A+h
∑f
i
∑f
i
u
i
=75.5+3×
30
4
=75.5+0.4=75.9
Hence, the mean heart beats per minute for these women is 75.9