15) \(\frac1{\sqrt a}-\sqrt c\), \(a=0.04\), \(c=0.64\)

\(\sqrt{0.04}=0.2\), \(\sqrt{0.64}=0.8\).

\(\frac1{0.2}-0.8=5-0.8=4.2\).

Ответ: 4.2