At noon joyce drove to the lake at 30 mph, but she made the long walk back home at 4 mph. how many hours did she walk if she was gone for 17 hours

By definition we have the following equation:
 t = d / v
 Where,
 t: time
 d: distance
 v: speed
 For this case we have:
 d / 30 + d / 4 = 17
 Rewriting we have:
 2d + 15d = 17 (60)
 17d = 17 (60)
 d = 60 mi
 Then, the walking time is
 t = d / v
 t = 60/4
 t = 15 hours
 
 She walked
 t = 15 hours