x/5=1/4 => x=5/4∉N deci: nu avem solutii in multimea nr. naturale
(nu are cum sa fie "adevarat" in multimea N !)
5/x=30/36 => x =(5*36):30 = 3 => x = 6 ∈N ;
4/12=x/6 => x=(6*4):12=2 => x=2∈N ;
18/6=3/x <=> 3/1=3/x => x=1∈N ;
24/6=4/x <=> 4/1=4/x => x=1∈N ;