AE/AB=AK/AD=EK/BD

So EF/BC=AK/AD

EF/ 120=AK/80

EF= 1.5AK

EF= 1.5*(AD-KD)

EF= 1.5*80- 1.5KD

Because EF=KD

So 2.5EF= 120 EF=48.

That is, the side length of a square is 48 mm