If someone who is 85% black and 10% white?

The convention many people use, and that some here disagree with, is that you are 1/2 of what each parent was. So, in your example, the child would be 42.5% black and 5% white from their mixed race parent, and 50% white from their white parent. You'd add the two white percentages for 55%.

no, not 10% it would be 5% which is 50%. because each sex during conception gives 50% of their dna.

So 50% of 85% black ( 42.5%), 50% of 10% white ( 5%) 50% of 5% other (2.5%) and mixed with  50% of 100% white (50%) results in a child who is .
42.5 % black, 55 % white and 2.5 % other.

A child could come out as black as charcoal or as white as any pure white, or anywhere in between. I have personally seen this myself within the family of a friend.

According to blood quantum law anyone that isn't white can claim any percent of any other blood type they want.

People do not come in bits, pieces fractions or percentages..... they have a mixed race child

It wouldn't make any difference to the person who is 85% white, but the child would be over half white, yes.

By the way, if the black parent is 85% black and 10% white, what's the remaining 5%? 

It doesn’t really work like this. There is an example, an Asian (mother) and an European (father) have a kid, a girl is brown haired with mixed facial features (eyes, noses, etc) but her brother is blonde with whiter skin and freckle, blue eyes but with Asian facial features. So it doesn’t just split into 50/50.

If you have a cup of coffee that is 10% milk, then mix it with another cup of coffee that's half milk and half coffee, what do you have?

When's the last time you took a Math class?

there's no such thing as percentages in genetic reality.

You have 46 chromosomes, that is 23 pairs, each pair consisting of one chromosome from your mother and one from your biological father. Any child you make will randomly have one chromosome out of every pair of yours. Most of the chromosomes do not influence skincolour at all

You can't say that someone is 85% back or 10% white. It doesn't have any meaning.

If you want analyze the genetic data, you must first remember that DNA is alike among human being. The level of similarity is 99.9%.

The influence of parents, grand-parents and so, is in the 0.01% zone.

Black or white are not scientific concepts.

You're either white or not.