To solve this problem, we can use the Pythagorean theorem to find the height of the balloon. In particular, we can consider the right triangle formed by the stake, the top of the balloon, and the part of the string that is attached to the stake.
[asy]
unitsize(2 cm);
pair A, B, C, D, E;
A = (0,0);
B = (0,15);
C = extension(A,B,A+(1,0),B+(1,0));
D = extension(A,C,A+(0,1),C+(0,1));
E = extension(B,D,B+(0,-1),D+(0,-1));
draw(A--B--C--cycle);
draw(A--D--E--cycle);
draw(rightanglemark(A,D,C,2));
label("$A$", A, S);
label("$B$", B, N);
label("$C$", C, E);
label("$D$", D, E);
label("$E$", E, W);
[/asy]
Let $a$ be the distance from the stake to the top of the balloon, $b$ be the height of the balloon, and $c$ be the length of the string. We know that $a = 15$ feet and $c = 15$ feet, so we can use the Pythagorean theorem to find $b$:
$a^2 + b^2 = c^2$
$15^2 + b^2 = 15^2$
$225 + b^2 = 225$
$b^2 = 0$
$b = \sqrt{0} = 0$ feet
Therefore, the height of the balloon is 0 feet. This is because the balloon has been pulled up by the wind so that its top is directly above the stake, and therefore its height is 0 feet.