Refer to http://www.r-tutor.com/gpu-computing/cuda-installation/cuda7.5-ubuntu for installing cuda 7.5 in ubuntu 14.04.

Begin from a fresh copy of ubuntu 14.0.4, without performing any upgrade.

Installing CUDA Toolkit 7.5 on Ubuntu 14.04 Linux

The following explains how to install CUDA Toolkit 7.5 on 64-bit Ubuntu 14.04 Linux. I have tested it on a self-assembled desktop with NVIDIA GeForce GTX 550 Ti graphics card. The instruction assumes you have the necessary CUDA compatible hardware support. Depending on your system configuration, your mileage may vary.

CUDA Repository

Retrieve the CUDA repository package for Ubuntu 14.04 from the CUDA download site and install it in a terminal.

From https://developer.nvidia.com/cuda-downloads, 

Download Target Installer for Linux Ubuntu 14.04 x86_64. As of 10 Nov 2015, the version is

cuda-repo-ubuntu1404-7-5-local_7.5-18_amd64.deb (md5sum: 5cf65b8139d70270d9234d5ff4d697c7)

Installation Instructions:

    sudo dpkg -i cuda-repo-ubuntu1404-7-5-local_7.5-18_amd64.deb
    sudo apt-get update

CUDA Toolkit

Then you can install the CUDA Toolkit using apt-get.

	sudo apt-get install cuda 

You should reboot the system afterwards and verify the driver installation with the nvidia-settings utility.

Environment Variables

As part of the CUDA environment, you should add the following in the .bashrc file of your home folder.
export CUDA_HOME=/usr/local/cuda-7.5 
export LD_LIBRARY_PATH=${CUDA_HOME}/lib64:LD_LIBRARY_PATH
 
PATH=${CUDA_HOME}/bin:${PATH} 
export PATH

CUDA SDK Samples

Now you can copy the SDK samples into your home directory, and build a test sample.
$ cuda-install-samples-7.5.sh  ~ 
$ cd ~/NVIDIA_CUDA-7.5_Samples 
$ cd 1_Utilities/deviceQuery 
$ make 

If everything goes well, you should be able to verify your CUDA installation by running the deviceQuery sample.