`a) \sqrt{16}.\sqrt{25}+\sqrt{196}:\sqrt{49}`
`=4.5+14:7`
`=20+2`
`=22`
`b) 36:\sqrt{2.3^2. 18}-\sqrt{169}`
`=36: \sqrt{2. 3^2. 2. 3^2}-\sqrt{13^2}`
`=36:18-13`
`=2-13`
`=-11`
`c) \sqrt{\sqrt{81}}`
`=\sqrt{9}`
`=3`
`d) \sqrt{3^2+4^2}`
`=\sqrt{9+16}`
`=\sqrt{25}`
`=5`