installed cuda 9.1 on ubuntu 16.04 but nvcc --version is showing version 7.5

i installed cuda 9.1 on my ubuntu 16.04 but when i check nvcc --version, the output is as follows:

…release 7.5, V7.5.17

why it is showing 7.5 when i installed 9.1

The message looks self-explanatory.
$PATH and most likely $LD_LIBRARY_PATH are pointing to a previous installation.