Using the answer in custom field (select) to auto fill other field
Depends on the context:

On a public or profile, develop a custom JS file to do this.

On editing a CiviCRM contact through the interface there's probably no good way to do this, other than adding custom JS in an extension.

If you're using Drupal Webform you can use the Validation Rules to show and hide fields. With some creative optioning you can get the results you need entirely through an interface.

My recommendation is the Drupal Webform.