Geometry
posted by Will .
Picture given with problem: media.glynlyon(dot)com/g_geo_2012/12/img_geou12abc_8.gif
If AC = 20, AE = 25, and AB = 5, what is the length of AD?
Please explain how this is found, I've got a few problems like this and have no idea how to do it.

You need to review properties of secants and circles. In your diagram,
AB*AC = AD*AE
5*20 = AD*25
AD = 4