The traveling salesman was dismayed...

[christopher.yh.chan]

6th Edition
Sentence Correction, Guide #8
p. 173
#6

Proposed Statement
The traveling sales was dismayed to learn that neither his sons nor his daughter were interested in moving.

Corrected Statement
The traveling sales was dismayed to learn that neither his sons nor his daughter was interested in moving.

My Question
P. 163 (indefinite pronouns) states "Either, neither (may require a plural verb if paired with or/nor". That is the case above but ended up as a singular verb. Is it correct to assume for cases using either/neither with or/nor is only plural if both subjects modified by or/nor both have to be plural to use a plural verb? If both subjects are not plural, then the verb should follow the closest subject?

[RonPurewal]

the most common convention is "closest subject".

...but, this has never been tested on the GMAT exam—and never will (since there is no universally accepted rule/convention for it).
so,
1/ you don't need to worry about this,
2/ it shouldn't be in the materials! (i'll follow up about having this removed from the next edition)

thanks.