An object is dropped from a platform 100 feet high. Ignoring wind resistance, how long will it take to reach the ground?

0  = -16t^2 + 100    subtract 100 from each side

-100  = -16t^2     divide both sides by -16

100/16   = t^2       take the positive root of both sides

10/4  = t   =  2.5 seconds