help with geometry

y / 20 = 4 / y

y² = 80

y = √80

@AboveGuest Very clever method... I didn't even think of using similar triangles!

Expanding on the solution, $\sqrt80$ can be simplified to $4\sqrt{5}$, so the number that goes in the green box is 4.

I like your answer too guest.

It was good that you left it not quite finished too.

I like that Eternity bought my attention to your solution too.  

I do not think of similar figure solutions as often as I should.  It is good to get reminders of them.