x_{17} = \frac{3}{2} - x_{16}\\
x_{21} = 15 - 2 \cdot \left(x_{1} + x_{3} + x_{6} + x_{9}\right) - \left(x_{2} + x_{4} + x_{5} + x_{7} + x_{8} + x_{10} + x_{11} + x_{12} + x_{13} + x_{14} + x_{15} + x_{16}\right)