Răspuns :
(d×4+20):4=12
=d×4+20=12×4
=d×4+20=48
=d×4=48-20
=d×4=28
=d=28:4
=d=7
Verificare:(7×4+20):4=(28+20):4
=48:4
=12
=d×4+20=12×4
=d×4+20=48
=d×4=48-20
=d×4=28
=d=28:4
=d=7
Verificare:(7×4+20):4=(28+20):4
=48:4
=12