please someone smart help to get me going, i dont know how to start these

"A 20 foot ladder is leaning against a building. The bottom of the ladder is sliding along the pavement directly away from the building at 1 foot per second. How fast is the top of the ladder moving down when the foot of the ladder is 5 feet away from the wall?"

i know this uses the pythagorean theorem somehow... i dont know what to do though