Compatibility of Mathcad 15 and Mathcad Prime 3.0 with Windows 10

Mathcad 15 is not yet fully compatible with Windows 7 or 8 . It will be with the next update though. I believe neither Mathcad 15 nor Prime 3.1 are officially compatible with Windows 10 (which does not mean that they are not, just that they have not been sufficiently tested yet). It's doubtful that Prime 3.1 will ever be officially compatible, but rather that Prime 4.0 will be.