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There is a possible problem with this:

The six numbers may add to more than the total population and the six percentages may add to more than 100 percent because individuals may report more than one race.

Shouldn't it be add UP to?

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Yes, because as it is, it really means the number will pile on top of existing values. In this context, that sense of add to (thereby increasing) does not make sense. What is intended is together amount to, which is add up to. This is only in the semantic sense. Grammar-wise the expressions are still valid, though. – Kris Nov 26 '12 at 11:21
I think "equal" could replace both instances and suffice. – tylerharms Nov 26 '12 at 13:46
In my opinion, both are OK. The numbers add up to 100. The numbers add to 100. Now, with multiply I would just say "The numbers multiply to 100" and not "multiply up". – GEdgar Nov 26 '12 at 15:16

Grammar isn't really the issue here. It's both pedantic and wrong to suppose numbers can only be "added to" other numbers, or "add up to" a total. It's true that in OP's context, "up" would normally be included - for example, the numbers add up to {some total X} is about four times more common than the numbers add to X, but that's just idiomatic preference. Consider...

He can add up two and two and get five. (17 instances)

OP might be happier if the word sum were used instead - I assume he has no problem with numbers that sum to a total (but there's nothing wrong with numbers that sum up to X either).

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Normally, people say "add to" when they mean to add to an existing or running total, and "add up to" when they mean to calculate a sum from scratch.

"Add the transaction amount to the balance."

"Add up the transaction amounts to calculate the balance."

In the example you give, the meaning is clear, so I wouldn't call it wrong. But it's unconventional. Yes, most English-speakers would write, "The six numbers may add UP to more than the total ..."

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