It told you it needed apache2-mpm-worker as a dependency to apache2. the first thing you should have tried is
sudo apt-get install apache2 apache2-mpm-worker
The only guess I have as to why it didn't do that automatically, is because there are several options for the Multi-Processing Module (MPM) package.
p apache2-mpm-event - Apache HTTP Server - event driven model
p apache2-mpm-itk - multiuser MPM for Apache 2.2
i A apache2-mpm-prefork - Apache HTTP Server - traditional non-threaded model
p apache2-mpm-worker - Apache HTTP Server - high speed threaded model
Here are all the packages installed for my apache2 server, as an example..
it-etch:/$ aptitude search ~i | grep apache2
i apache2 - Apache HTTP Server metapackage
i A apache2-mpm-prefork - Apache HTTP Server - traditional non-threaded
i A apache2-utils - utility programs for webservers
i apache2.2-common - Apache HTTP Server common files
i A libapache2-mod-php5 - server-side, HTML-embedded scripting language
Hope that helps.