$\begin{array}{l} b)\, (\sqrt{20} + \sqrt{45} - \sqrt5).\sqrt5\\ =(\sqrt{4.5} + \sqrt{9.5} - \sqrt5).\sqrt5\\ =(2\sqrt5 + 3\sqrt5 - \sqrt5).\sqrt5\\ =4\sqrt5.\sqrt5\\ =4.5=20\\ d)\, (\sqrt6 + 2)(\sqrt3 - \sqrt2)\\ =\sqrt2(\sqrt3 + \sqrt2)(\sqrt3 - \sqrt2)\\ =\sqrt2(\sqrt3^3 - \sqrt2^2)\\ = \sqrt2(3 - 2) = \sqrt2 \end{array}$