"While we know that the set of real numbers is uncountable, the set of computable numbers is countable, and thus we know that most real numbers are not computable."
From Wikipedia: "Consider a system of n linear equations for n unknowns, represented in matrix multiplication form as Ax = b where the n × n matrix A has a nonzero determinant, and the vector x = {x_1, x_2, ..., x_n}^T is the column vector of the variables. Then Cramer's rule states that in this case the system has a unique solution, whose individual values for the unknowns are given by: x_i = det(A_i)/det(A)"