set it up as a ratio:
mans shadow over mans height (4/6 ) equals flag pole shadow over flag pole height (15/x)
set them to equal each other:
4/6 = 15/x
cross multiply: 4*x =4x, 15*6 = 90
so you have 4x = 90
now divide both sides by 5 to solve for x, which is the height of the flag pole:
x = 90 / 4 = 22.5 feet tall