We have three similar triangles, let's see if we can get this right, right vertex first, then other vertex from small side:
EDF ~ GDE ~ GEF
As ratios that means
ED:DF:EF = GD:DE:GE = GE:EF:GF
x:20:y = 5:x:GE = GE:y:15
Since we're after y we choose
20/y = y/15
20(15) = y²
y² = 300
y = 10√3
y ≈ 17.32
