[tex] \sqrt{ \frac{81}{400} } = \frac{ \sqrt{81} }{ \sqrt{400} } = \frac{9}{20} [/tex]
[tex] \sqrt{ \frac{16}{25} } = \frac{ \sqrt{16} }{ \sqrt{25} } = \frac{4}{5} [/tex]
[tex] \frac{9}{20} + \frac{4}{5} = [/tex]
[tex] \frac{9}{20} + \frac{16}{20} = \frac{16 + 9}{20} = \frac{25}{20} = \frac{5}{4} [/tex]