E: Version '2.0.25' for 'mongodb-10gen' was not found while installing this specific version
if anone lands here in future then follow these steps for installing OLDER versions ( 2.4 and earlier versions )

  sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10

        echo 'deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen' | sudo tee /etc/apt/sources.list.d/mongodb.list

        sudo apt-get update

    apt-get install mongodb-10gen=2.0.2


replace 2.0.2 with your required version.

for Versions 2.6 and above replace last step with this format 

sudo apt-get install -y mongodb-org=2.6.9 mongodb-org-server=2.6.9 mongodb-org-shell=2.6.9 mongodb-org-mongos=2.6.9 mongodb-org-tools=2.6.9


replace 2.6.9 with your mongo version ( only 2.6 and above )