20 16 there is a unique answer to the 23 questions in the second entrance examination for mathematics. 20 16 Postgraduate Mathematics II 23 is a topic about linear algebra. When the coefficient determinant of homogeneous linear equations is zero, there are multiple solutions (or no solutions), then there are non-zero solutions. But if the determinant is not 0, there is a unique solution, that is, an all-0 solution, and there is no non-zero solution.