Answer question no3 by direct mean formula
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Addition of 7 + 6 + 9 + 13 + f + 5 + 6 = 44 + f ----- (ii)
11-13 xi = 11 +13 /2 = 12, fixi = 12 * 7 = 84
13-15, xi = 14, fixi = 84
15-17, xi = 16, fixi = 144
17-19, xi = 18, fixi = 234
19-21, xi = 20, fixi = 20f
21-23,xi = 22, fixi = 110
23-29, xi = 24, fixi = 96
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752 + 20f ------------- (iii)
Substitute (ii) and (iii) values in (i)
18 = 750 + 20f/44 + f
on cross multiplication we get
792 + 18f = 752 + 20f
f = 20.