Question 15533
Let the positive number be n.

{{{sqrt(n) = n/40}}} Square both sides.
{{{n = n^2/1600}}} Multiply both sides by 1600.
{{{1600n = n^2}}} Divide both sides by n.
{{{1600 = n}}}

The positive number is 1600.

Check:

{{{sqrt(1600) = 1600/40}}}
{{{40 = 40}}}