Fie ab barat numarul nostru:
ab : ba = 2 rest 18 ==> ab = 2 * ba + 18
10a + b = 2(10b + a) + 18
10a + b = 20b + 2a + 18
19b - 8a + 18 = 0
Stim ca a + b = 9 ==> b = 9 - a
Inlocuim:
19(9 - a) - 8a + 18 = 0
171 - 19a - 8a + 18 = 0
189 - 27a = 0
27a = 189
a = 189/27 = 7 ==> b = 2 ==> ab = 72