apt - unmet dependency error on ubuntu 14.04 while installing

whenever I am trying to install php5 or my-sql I am getting below error.

The following packages have unmet dependencies:
mysql-server : Depends: mysql-server-5.5 but it is not going to be installed
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).

What's the problem here.