I saw this sentence in the math textbook.
For example, if we choose two 2s, zero 3s, and one 5, we get the divisor
Here they said zero 3s and one 5. Two is plural and one is singular, which is obvious, but zero is considered as plural so they said 'zero 3s'. Why is zero plural?
0 = plural, 1 = singular, 2 = plural, 3 = plural, and so on...
--> This relation does not look natural for me.
That is just how the language works. All numbers except exactly 1 are plural. Note that -1 can also be singular, but that depends on the context and dialect.
Examples
Please have a look at this answer from English Stack Exchange
Idiomatically, a quantity is singular if and only if it is being regarded as a single entity (including a fractional part of a whole) and is not in decimal form:
1 litre is left
¾ (of a) litre is left
three-quarters of the cake is left.
five-fourths of the cake is left.
1.0 litres (e.g., 1.003 litres to 1 decimal place) are left
one point zero zero dollars
0.75 litres are left
0 litres are left
1¾ litres are left
three-quarters of the pieces of cake are left
three quarters of the cake (three quarter-cake slices) are left.
This convention for quantities, which are by definition nonnegative, holds regardless of whether the unary operation ‘minus’ is being applied. So, “-1 litre” and “-0.75 litres” (whatever these mean) are correct.
When a plural noun is being regarded collectively, it agrees with a singular verb:
The choice between “half the crew is late” and “half the crew are late” depends on the intended emphasis.
Note that “I have no legs” is alluding to the number 0 (as opposed to specifying 1 leg or 2 legs or any other number of legs), whereas “I have no leg” is noting the absence of a leg (i.e., saying that I have not one leg).
Zero is not plural. The plural of zero is "zeroes" as in "The score for the team today is zeroes all across the board." (Baseball)
But when zero is used as an adjective to modify something countable the word it modifies is expressed in the plural. Examples from other answers are "zero books", "zero liters", and your own example, "zero 3s".
The reason for this is that zero is not one. In English usage, all numbers but one, when used as modifiers, throw the noun that is modified into the plural. We have no way to express a zero amount of a thing, only a singular way and a plural way, and since it is not singular, it is plural.
This is true also in other words that express negation. We say, "There are no ducks on the pond" and "None of the light bulbs are working." There is no noun form to represent a zero number, only singular and plural.
It seems natural to me. Note that you say “I met no cats today.” No cats, one cat, two cats...
(Though in the Lord of the Rings, Eowyn says “I am no man.” https://www.youtube.com/watch?v=W7_c-R7i8F4)
The number zero was invented against a backdrop of numbering systems which don't have placeholders. So instead of "nothing in the tens place" they would just omit that place entirely and provide a different kind of thing instead. Roman numerals do this, they just make 10 and 100 fundamentally different by using X or C instead.
Along these lines, knots have been used as a system of accounting. The idea is that you'd have a container full of things, and tied to it would be a string. Knots on that string would tell you how many things are in there, which might be easier than cracking the lid and counting, especially if it's something like cups of rice.
My point is that when you're doing this kind of accounting, you physically group the objects into recognizable shapes (likely rectangles of consistent side length). But then if you're using a numbering system with 0 in it, you don't directly correspond the shapes to symbols, you have to count the number of same-type shapes you have. So you end up asking:
how many of that shape do I have?
So there's this implicit potential for "many", even if it might turn out that there's only one in your particular case. I imagine a proponent of the new system, explaining it to one of the old guard. They would have them group the objects into piles of certain shapes/sizes and at some point say:
If you don't have 1 or 2 or 3 ... or 9 tens then you put a zero here.
Sample language sentence:
"For example, if we choose two 2s, zero 3s, and one 5, we get the divisor [math formula]: 2^2 x 3^0 x 5^1 = 20."
The sentence preceding the equation has English mistakes in it. It does not accurately or correctly describe the equation below it (in the question).
Rewrite: If we write 2 to the power of 2, 3 to the power of 0 and 5 to the power of 1, we get: 4 x 1 x 5 = 20.
zero is not plural there, in that equation.
Question: Why is zero plural? Answer: There is no plural zero in the equation or in the incorrectly written description of it.
Singular: zero Plural: zeros
The problem is that "s" designates a plural, while "apostrophe (”'")s" represents a possessive. In the example offered, "3's" represents a category of numerals characterized by the symbol 3 have a property that they possess of zero or one or two that is, no 3's, one "3" (3), or two "3's" (3,3).
