Prompted data entry

[Jill]

I need to prompt the user for data entry as follows:

user will enter data to the following cells: F24, F25, F26 and F27.
cell F24 includes a drop down list that users can pick the data.

Sometimes users enter data to F25, F26 and F27 without entering the data in
F24 (it is left as blank).

How can I prompt users to enter data to F24 when they enter data in F25, F26
and F27? In other words, F24 cannot be left blank if there are data in F25,
F26 and F27.

Please help!!

Regards
Jill