x_{21} = \left(x_{18} + x_{7} + x_{19} + x_{8}\right) / 2\\ x_{22} = \left(x_{18} + x_{7}\right) \cdot k_{16}\\ x_{23} = \left(x_{19} + x_{8}\right) \cdot k_{17}\\ x_{24} = \left(x_{7} + x_{8}\right) / 2 \cdot k_{18}\\ x_{25} = x_{7} \cdot k_{19}\\ x_{26} = x_{8} \cdot k_{20}\\ x_{27} = x_{20}\\ x_{28} = \left(x_{19} + x_{8}\right) / \left(x_{18} + x_{7}\right)