While PTC has moved on to (versions 6–10+), a significant portion of the engineering world still clings to Mathcad 15 . Why? Power, flexibility, and the classic interface.
If you already own a license for Mathcad Prime (version 7 or later), you are technically entitled to use Mathcad 15. You can often find the installer through the PTC eSupport Portal if you have an active maintenance agreement. ptc mathcad 15 download