a) `5\sqrt{18}-\sqrt{50}+\sqrt{8}`
`=5\sqrt{9.2}-\sqrt{25.2}+\sqrt{4.2}`
`=15\sqrt{2}-5\sqrt{2}+2\sqrt{2}`
`=12\sqrt{2}`
b) `(2\sqrt{6}+\sqrt{5})(2\sqrt{6}-5)`
`=2\sqrt{6}.2\sqrt{6}-10\sqrt{6}+2\sqrt{30}-5\sqrt{5}`
`=24-10\sqrt{6}+2\sqrt{30}-5\sqrt{5}`
c) `(\sqrt{20}-3\sqrt{10}+\sqrt{5})\sqrt{5}+15\sqrt{2}`
`=\sqrt{100}-3\sqrt{50}+\sqrt{5}.\sqrt{5}+15\sqrt{2}`
`=10-15\sqrt{2}+5+15\sqrt{2}`
`=15`