  We establish a `mixed' version of a fundamental theorem of Khintchine within
the field of simultaneous Diophantine approximation. Via the notion of ubiquity
we are able to make significant progress towards the completion of the metric
theory associated with mixed problems in this setting. This includes finding a
natural mixed analogue of the classical Jarn\'ik-Besicovich Theorem. Previous
knowledge surrounding mixed problems was almost entirely restricted to the
multiplicative setup of de Mathan & Teuli\'e [21], where the concept
originated.
