How Do You Calculate Displacement at Reduced Velocity in Free-Fall Motion?

[The Eggman]

Homework Statement

A stone is thrown straight upward with a velocity of 10V and travels upward a distance X. What is the displacement when the object is traveling at 5V?

Homework Equations

xf=x0+v0t+1/2at^2

vf=v0+at

vf^2=v0^2+2ax

The Attempt at a Solution

.75X? Further, what's the most efficient way of solving for it?

 

[Spinnor]

With half the speed the stone has 1/4 of the kinetic energy left so can only go 1/4*X higher or it is at 3/4*X when the speed is 5V.