Relations between modules: automate countdown operation.

Hey, everyone!

I have two modules:

  1. Job

  2. People

  3. “Job” is module for describing different kind of jobs. It contains fields like Title, City, Bank account and other. There are two important fields:

    1.1 No. of Vacancies: manual entry
    1.2 No. of Candidates: manual entry

  4. “People” is module for managing people who are seeking the job.

When administrator found some people who are ready to work, he choose the name in the “Job” module, in the tab “People”.

The problem is that field “Vacancies” after administrator chose the people doesn’t changes (normally it should doing minus one).

For example:

Job called “Doctor” with 5 Vacancies. When Administrator has found 2 people, fields in Vacancies should be 3 (it should automatic countdown).

My questions:
Is it possible to configure like I’ve described: with automatic countdown when some people is checked?
What is the best practice?

Am I right that I need to code it by myself using logic_hooks.php ?

Or someone now solution without PHP ?