a : b = 9 rest 7
c : b = 9 rest 8
a = 9b+7
c = 9b+8
a +c =( 9b+7) + (9b +8)
= 18b +15, unde b = 1,2,3,4,5,6,7,8,9
a+c = 18 x 1 +15+
18 x 2 +15
18 x 3 +15
18 x 4 +15
18 x 5 +15
18 x 6 +15
18 x 7 +15
18 x 8 +15
18 x 9 +15 = 18 x (1 +2 +3 +4 +5 +6+7 +8+9) +9 x 15
=18 x 45 + 9 x 15
= 810 +135
= 945