11-(7x5-9x9:f:3):8=7
⇔11-(35-81:f:3):8=7
⇔(35-81:f:3):8=11-7
⇔(35-81:f:3):8=4
⇔35-81:f:3=8x4
⇔35-81:f:3=32
⇔81:f:3=35-32
⇔81:f:3=3
⇔81:f=3x3
⇔81:f=9
⇔f=81:9
⇔f=9
proba:11-(7x5-9x9:9:3):8=
=11-(35-81:9:3):8=
=11-(35-9:3):8=
=11-(35-3):8=
=11-32:8=
=11-4=7
