Abuilding 50 ft high casts a 75-ft shadow. sarah casts a 6-ft shadow. the triangle formed by the building and its shadow is similar to the triangle formed by sarah and her shadow. how tall is sarah?

Make each known side into a ratio like so...
75/6 
(Because 75 is the shadow casted by the building, and 6 is the shadow casted by sarah)
and 50/x 
(50 is how tall the building is and sarah's height is what needs to be found) 
Therefore, 
75    50
 
 6      x

Do cross products;
50 x 6 = 75 x X
300 = 75x 
Divide by 75 
x = 4ft