The number is 1/25.
Solution:
Let x be number:
[tex]x ^{-1/2} =5[/tex]
[tex] \frac{1}{ x^{1/2} } =5[/tex]
[tex] \frac{1}{ \sqrt{x} } =5[/tex]
[tex]( \sqrt{x}) \frac{1}{ \sqrt{x} } =5( \sqrt{x} )[/tex]
[tex]1 = 5 \sqrt{x} [/tex]
[tex](5 \sqrt{x} ) ^{2} =(1) ^{2} [/tex]
25x = 1
25x = 1
25 25
x = 1/25