The length of the shadow on flat ground of a man who is 6 feet tall and is standing up straight is 8 feet. the distance between the top of his head and the top of his head in the shadow is feet.

The answer is 10 ft.  make a triangle 6ft tall, 8ft long (90 degrees to the other line) and draw a line to complete the triangle.  This is a type of 3,4,5 triangle, a 6,8,10.  The other way to do it is sqrt(6^2+8^2) = sqrt(36+64) = sqrt (100) = 10.