Answer by zhenguoli for How can I have tomcat and apache running at the same time?

The port 80 have been binded to the tomcat, so when you start apache, it can't bind to the port 80 which is the default port of apache.

So you should change the apache default port in /etc/apache2/ports.conf file as
Listen 8081
Then you can verify that Apache listing:
sudo netstat -tulpn | grep :8081