First let's draw in this right triangle to
indicate the angle of depression to the top
of the shorter building from the top of the
tall building:



The distance between the buildings is X, and that's
what we want to find.

From that right triangle we get



Multiply both sides by 



or



Next redraw the figure, but this time let's 
draw in this right triangle to indicate the 
angle of depression to the bottom of the 
shorter building from the top of the taller
building:



From that right triangle we get



Multiply both sides by 



or



Now we put those two equations together:



We set the right sides equal, since they both 
equal to :



or



Factor out  on the left



Get your calculator and find this value: 



which is , so we substitute this 
in the bracket:



Divide both sides by 







or about  to the nearest ten feet.

Edwin