Help please A young tree casts a lunchtime shadow of 4.2 feet. The 3 foot mail box casts a late afternoon shadow of 2.8 feet. How tall is the young tree?
(Hint: draw triangles and label them)

Triangle BDC is an isosceles. 
So, angle DBC = angle DCB.

180° - 100° =80°
80÷2=40.

Angle C = 40°

Angle ABD is half angle DBC, so <ABD = 40°÷2=20° 
Therefore, angle ABC=20°+40°=60°
To get the angle A, use the angles in a triangle ABC.

180° - (60°+40°)=80°

Angle A = 80°