The following packages have unmet dependencies: nodejs : Conflicts: npm
Solution 1:
The nodejs package you installed from the PPA includes npm since v0.10.0. Since you have v0.10.15, when you ran sudo apt-get install nodejs you also installed npm. Try to run npm to confirm.
Also see the offical installation guide (the one you followed is a copy of it, but the official one has a few extra details).