--- doc/build/ubuntu20LTS_install.frag	2021/03/17 13:37:40	1.4
+++ doc/build/ubuntu20LTS_install.frag	2022/03/26 19:00:51	1.5
@@ -137,13 +137,29 @@ sudo apt-key add APT-GPG-KEY-loncapa.asc
 <p> Execute: </p>
 <pre>
 sudo apt-get install software-properties-common
-sudo apt-add-repository 'deb http://install.loncapa.org/ubuntu focal main'
+sudo add-apt-repository 'deb http://install.loncapa.org/ubuntu focal main'
 </pre>
 <p>
 You can display which repositories are currently enabled by using:
 </p>
 <pre>
-sudo apt-cache policy
+grep '^deb ' /etc/apt/sources.list |grep ' focal '
+</pre>
+<p>The output should contain:</p>
+<pre>
+deb http://us.archive.ubuntu.com/ubuntu focal main
+deb http://us.archive.ubuntu.com/ubuntu focal universe
+deb http://install.loncapa.org/ubuntu focal main
+</pre>
+<p>If the Ubuntu universe repository is absent it can be added using:</p>
+<pre>
+sudo add-apt-repository universe
+</pre>
+<p>If other Ubuntu repositories, e.g., restricted and multiverse are enabled, 
+you can disable them, as they are not needed for LON-CAPA, by using:</p>
+<pre>
+sudo add-apt-repository --remove restricted
+sudo add-apt-repository --remove multiverse
 </pre>
 <h3>Install prerequisites</h3>
 <p> Execute: </p>