  A new geometric representation of qubit and qutrit states based on
probability simplexes is used to describe the separability and entanglement
properties of density matrices of two qubits. The Peres--Horodecki positive
partial transpose (ppt)-criterion and the concurrence inequalities are
formulated as the conditions that the introduced probability distributions must
satisfy to present entanglement. A four-level system, where one or two states
are inaccessible, is considered as an example of applying the elaborated
probability approach in an explicit form. The areas of three Triadas of
Malevich's squares for entangled states of two qubits are defined through the
qutrit state, and the critical values of the sum of their areas are calculated.
We always find an interval for the sum of the square areas, which provides the
possibility for an experimental checkup of the entanglement of the system in
terms of the probabilities.
