explain echo method to find the distance of the moon

In this method, laser beam is used in place of sound waves to find the distance of moon from earth.
A laser beam is sent towards moon. This transmitted beam from earth is received back on earth after reflection from the moon.
Suppose the time interval between transmission and reception of the beam is t, the velocity of beam in air/vacuum is c and s is the distance of moon from earth,
Then
Distance=Velocity x time
2s=c x t
 or s= ct/2