For questions like these you should draw the dimensions as a right triangle and use sine, cosine, and tangent to determine the answer. In this case, tangent is our friend.
The angle we need is equal to
[tex] \frac{35}{60} [/tex]
which is the tree height divided by the shadow length. All we have to do is inverse the function and do
[tex] \arctan( \frac{35}{60} ) = 30.256[/tex]
