This question already has an answer here:
I have just seen some answers here to the effect that
Ten percent of the pie is mine
This seems rather odd to me. Ten percent is a plural subject, so why should it have a singular verb.
Note that, grammatically, of the pie is subordinate to percent and should be inessential; that is, it should be possible to remove it without affecting the grammaticality of the sentence.
My question is not whether the example sentence is correct (that is the question of which this is alledgedly a duplicate) but why it is considered correct -- given my grammatical objection -- and for how long it has been. I would guess that this is a relatively new phenomenon and that in the good old days, there was no special rule for this case, such that the verb would have had to be are.
Please do not mark this question as a duplicate without reading it. How is it possible to mistake a question asking why something is considered correct and for how long with one asking what is correct?