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When the number of individuals are already mentioned derive the frequency in 100 individuals: for this question which will be as following --

Number of homozygous dominant 49% ( $B^2$)
Number of heterozygous dominant 42% ( $2Bb$)
Number of homozygous recessive 9% ($b^2$)

(There are two alleles B and b)

Now you can derive the frequency of alleles:

Frequency of allele B = $sqrt(.49)$ = .7
Frequency of allele b = $sqrt(.9)$ = .3

(According to Hardy Weinberg principle --
sum of all the frequencies of alleles = 1
in this case .7 + .3 = 1 )

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