The longer an object, the longer the shadow they cast at a given time of the day and vice versa. This is called direct proportionality. At the same time,
If 15 foot flagpole ---- casts 11 foot shandow
  28 foot tree -------- casts ?? foot shadow
The proportion would therefore be:
Length of shadow cast by tree = (28/15)*11 = 20.533 foot.
