Let us say that:
a = black kittens
b = white kittens
We know that:
a = 5 b
a + b = 25
Therefore:
5 b + b = 25
6 b = 25
b = 4.17
But we cannot have a fraction value for the number of kittens, so round off:
b = 4
So a is:
a = 25 – 4 = 21
Hence Zoe counts additional 1 black kitten.