Percentages - singular or plural when it's a % of a group of items:

Question

Example : 50% of paper and board produced globally is/are used for packaging?

Answer 1

You need to treat fifty percent of the same way that you treat a lot of and a number of: as a mere premodifier of the noun that follows it, one that works like determiners and adjectives.

That way the actual head noun of that noun phrase is not percent, lot, or number, but the thing that comes after it, and that noun’s grammatical number is unaltered for purposes of subject–verb agreement.

For example:

• Your problem is that you planted too much zucchini.
• A lot of your problem is that you planted too much zucchini.
• Some of your problem is that you planted too much zucchini.
• Half of your problem is that you planted too much zucchini; the other half is that you eat too little of it.
• Fifty percent of your problem is that you planted too much zucchini; the other fifty percent is that our town doesn’t have a Little League team.

In all those scenarios, the head noun of the noun phrase functioning as the grammatical subject remains problem, which therefore requires is as the verb.

Here is a contrasting situation with a plural head noun:

• These apples have worms in them.
• A lot of these apples have worms in them.
• A number of these apples have worms in them.
• Many of these apples have worms in them.
• Half of these apples have worms in them.
• Fifty percent of these apples have worms in them.

In all cases above, apples remains the subject; the predmodifiers do not change that, and so the verb remains have in the plural.

Answer 2

Normally, for items, one would say 50% is used.
This is because a fixed percentage of something is, in itself, a singular concept.

However, this is not always the case when referring to people. You will regularly see usage such as 10% of mothers are claiming benefits.

Answer 3

Think of it by how you would refer to the subject. They or it? In this case 50% of paper would be 'it'. In Chenmunka's example of mothers claiming benefits, you would refer to them as 'they'. So, follow the rule of 'it is' and 'they are'.

Answer 4

50% of paper and board is used for packaging.

I believe that this may apply: "is" vs. "are" and "it" vs "they" are differentiated by the divisibility of the subject; i.e., one piece of paper is infinitely severable, whereas one person is an irreducible unit.