Obviously, the image formed by the big hole is symmetrical about y, so you can set y =-kx 2+6.

Because ab = 20 meters

So b (10/0,0) is k = 0.06.

M (0,6) When y=4.5, the value of x can be found to be 2.5 meters.

That is, the distance of EF is 5 meters.

I wish you a happy study!