ab-numar natural ,unde a,b∈N;⇒rasturnatul lui ab este ba-numar natural ,unde b,a∈N;
⇒ab=10a+b;
⇒ba=10b+a;
⇒ab/ba=4/7⇒10a+b/10b+a=4/7;
⇒7[10a+b]=4[10b+a]
⇒70a+7b=40b+4a⇒70a-4a=40b-7b=66a=33b⇒2a=b;
⇒b=2a⇒ab∈[12,24,36,48]⇒4 numere naturale de doua cifre vare indeplinesc conditiile problemei;