a : 6 = 13 rest 0
a = 13 × 6 + 0
a = 78
b : 6 = 13 rest 1
b = 13 × 6 + 1
b = 78 + 1
b = 79
c : 6 = 13 rest 2
c = 13 × 6 + 2
c = 78 + 2
c = 80
d : 6 = 13 rest 3
d = 13 × 6 + 3
d = 78 + 3
d = 81
e : 6 = 13 rest 4
e = 13 × 6 + 4
e = 78 + 4
e = 82
f : 6 = 13 rest 5
f = 13 × 6 + 5
f = 78 + 5
f = 83