The middle angles of a rhombus are 90 degrees which means the other two angles of the triangle must total 90 degrees.
(8x +7) + (3x + 6) = 90
11x + 13 = 90
11x = 77
x = 7
as you already know, to get the inverse of any expression we start off by doing a quick switcheroo on the variables and then solving for "y", let's do so.

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3 boys for every 2 girls
So, the percentage of girls is (2 / 5) x 100 = 40%