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"Jim and John have strong right arms."
I believe the usage of plural form of "arm" here is correct. Now, I am confused about we should use singular or plural form of "digit" in the following sentence.
"All multiples of 10 have 0 as their units digits."
How about this?
"All even numbers have even units digits."
Also, please explain the grammar rule along with the answer.
You should use singular in this case.
There is only one units digit in each multiple, and it is 0.
Much the same can be accomplished in any complex network:
The difference between the singular has and plural have is because every is singular, while all is plural (I don't make up these rules). But there is only one supervisor in each case, and it's an accountant who's the supervisor.
Aside from how this is formalized in grammar rules, you aren't comparing like with like. You'd say "John and Jim both have a hook for their right hand", not "John and Jim both have a hook for their right hands". Or you could say "John and Jim have hooks for right hands".
Similarly, you can say "All multiples of ten have zero as their units digit", or you can say "All multiples of ten have zeroes as their units digits".
So the answer to your question is that you can phrase this either with singular or with plural, but you don't mix the two.
"All even numbers have even units digits" and "All even numbers have an even units digit" are both grammatically correct. You can dispute whether one of them suggests more strongly than the other, that a number can potentially have more than one units digit. But since we know that's not the case, no such difference in meaning arises in practice for that example.
Applying the same variations to John and Jim: you can say "John and Jim have strong right arms", but you would normally avoid "John and Jim have a strong right arm". It's grammatical and might make sense in context, but the construction rather suggests they have one between them instead of one each, as you might say "John and Jim have a labrador".
You can say "All men have a strong right arm" (not true, but it parses). The implication of one arm between them could still be drawn from that, but it's much more of a stretch, it's not the "usual" meaning. So there's a difference between saying "all" and providing a list.
