There're five black-furred mice, and five white-furred mice, which are the offspring of a black and a white mouse.
We know that the allele for black fur is B, while the allele for white fur is b, and only if there're two recessive alleles for a trait, it would be expressed, in this case, bb will be the genotype of the white mouse, while the genotype of the black mouse can be BB or Bb.
We know that the total number of the offspring is 10 and that the given number of black fur individuals of the offspring is 5, and so the given number of white fur individuals, so the given percentage could be calculated as follows:
We can see that the percentage of those individuals will be 50 % for each fur color, and then, the genotype of the black-furred parent mouse is Bb, while the genotype of the white-furred parent mouse is bb.