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Find the arithmetic mean of:

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`sum_(i=1)^(5)f_(i)x_(i) = f_(1)x_(1) + f_(2)x_(2) + f_(3)x_(3) + f_(4)x_(4) + f_(5)x_(5)`
` = 3 xx 10+4 xx 15 + 2 xx 20 + 5 xx 25 + 6 xx 30`
`=435`
`sum f_(i) = 3+4+2+5+6 = 20`
`"Mean" = (sum f_(i)x_(i))/(sum f_(i)) = (435)/(20) = 21.75`
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