Basically.. when you draw a line from the vertex of a right triangle's 90 degree angle to its hypotenuse, it'll make two triangles with the same angles. Then you can use ratios to discern the size of said rectangles. I hope that helps.
850. If there is 17 red beads for every 150 beads Then you would divide 7500 by 150 and you get 50. Then multiply 50 with 17 and your answer is 850 red beads. 7500/150=50. 50 x 17= 850.