ab:ba=4 rest 15
ab=4ba+15 ab=10a+b ; ba=10b+a
=>10a+b=4(10b+a)+15
10a+b=40b+4a+15
6a-15=39b impartim fiecare termen la 3
2a-5=13b
b<2 (daca b ar fi 2=> 2a-5=26 , dar a este cifra )
deci, b=1 => 2a-5=13=>2a=18 =>a=9
=> ab=91
Proba: 91:19=4 rest 15; 19•4+15=76+15=91