The question states that this point is on segment , with the distance between this point and being the distance between to .
It could be shown (using a pair of similar right triangles) that the ratio applies not only to the (sloped) distance between this point and , but to the vertical distance as well.
The vertical distance between this point and would also be the vertical distance between and .
The vertical distance between and is the difference between their coordinates, .
The vertical distance between this point and would be of the vertical distance between and , .
Since is above , any point on the segment between these two points would also be above . Add the vertical distance between and the requested point to the coordinate of to find the coordinate of the requested point: .
300/2000 is a fraction but simplified it's 3/20. In a decimal, you need to get the denominator to 100 so multiply it by 5 to get 15/100, which equals 0.15, and in a percent it'd be 15%.