I suppose you could think of this like mixing paint. I have 100 units of a paint mixture that is make up of 75 units of blue paint and 25 units of blue paint. I have another pail with 100 units of blue paint. If I mix them together I have 200 units of paint, 75 units of which are green and 125 units of which are blue. Thus the resulting mixture is
75/200 100 = 37.5% green and 125/200 100 = 62.5% blue.