What is the easiest way to install numpy with LAPACK/BLAS?

In Ubuntu 14.04 and later, blas and lapack are installed as part of python-scipy and python3-scipy, so you need to install python-scipy. In Ubuntu 18.04 and earlier open the terminal and type:

sudo apt install python-scipy  

This command will also install libblas3 (Basic Linear Algebra Subprograms reference implementations, shared library) and liblapack3 (Library of linear algebra routines 3 - shared version) as dependencies, and it will also install python-numpy as a dependency if you don't already have it installed.