Say I have one old and one new bike.
Should I write:
Both the old and the new bike should be there.
or
Both the old and the new bikes should be there.
Say I have one old and one new bike.
Should I write:
Both the old and the new bike should be there.
or
Both the old and the new bikes should be there.
The first is more specific: you have one of each.
The second is less specific: you have a mixture of old and new bikes.
Both the old bike and the new bike should be there.
The old bike is singular
The new book is singular
So the old bike and the new bike are plural and mean one old book and one new book.
So the above sentence is correct.
If you say :
Both the old and the new bikes, it means some old bikes and some new bikes.
It is not what you intended to say