[ 8(a) + b ] + 5[ a + b] = 13a + 6b
13a + 6b = 8b + a
12a = 2b
6a = b
a cannot be an integer > 1 because this would make b at least 12....which is impossible in base 8
So...
a = 1 and b = 6
Proof
168 + 5 [ 1 + 6] = [ (1)*8 + 6] + 5*7 = [ 14 ] + 35 = 49
And
618 = 6(8) + 1 = 49
So
N = 168
Oh, I see. I thought we had to add the digits when solving in base 8, so I got really confused. Thanks a lot for your help!!! I like the proof!