a2b (barat)=2ba (barat) ; a≠0 ; trecand la scrierea in baza 10 ⇒
100a+10·2+b=100·2+10·b+a ⇔
100a+20+b=200+10b+a |-a ⇔
99a+20+b=200+10b |-10b ⇔
99a+20-9b=200 |-20⇔
99a-9b=180 |:9 ⇔
11a-b=20
Daca a=1 atunci 11·1-b= 20 ⇔11-b =20 ⇒ caz imposibil (nu exista cifre negative)
Daca a=2 atunci 11·2-b= 20 ⇔22-b=20 ⇔ b=2 ⇒ab (barat)=22
Daca a=3 atunci 11·3-b= 20 ⇔33-b=20 ⇔ b=13 (nu exista cifra 13 in baza 10)
