Rearrange the formula for the kinetic energy, k = \frac{1}{2} m v^2, to isolate v, the velocity.

V = The square root of (2K/M)

We are starting with the formula: K = 0.5MV^2

Our first steps to isolate V would be to multiply by 2 and divide by M on both sides of the equation.

2K/M = V^2

Now, we just have to take the square root of both sides of the equation.

In this problem it decreased 20%