What percentage will be brown if black rabbit homozygous mates with a rabbit?

The genotype of the black homozygous rabbit is BB, and the genotype of the brown rabbit is bb. When they mate, the offspring will be Bb, which is heterozygous for coat color. Heterozygous rabbits have one copy of the dominant allele (B) for black coat color and one copy of the recessive allele (b) for brown coat color. Because the black allele is dominant, all of the offspring will have black coats. None of the offspring will be brown.

Therefore, the percentage of brown rabbits in the offspring of a mating between a black homozygous rabbit and a brown rabbit is 0%.