I am new to ROS and Gazebo. And I am having trouble installing Gazebo packages on Ubuntu 16.04

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I am following this link to install gazebo's dependencies.

When I run this command sudo apt-get install -y libgazebo7-dev, I get this error:

[sudo] password for cl201-9: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
libgazebo7-dev : Depends: libsdformat4-dev but it is not going to be installed
                  Depends: libgazebo7 (= 7.15.0-1~xenial) but it is not going to be installed
                  Depends: gazebo7-common (= 7.15.0-1~xenial) but it is not going to be installed
                  Depends: gazebo7-plugin-base (= 7.15.0-1~xenial)
E: Unable to correct problems, you have held broken packages.

How can this be fixed?