abc : bc = 6 rest 5
abc=6•bc+5
100a+bc=6•bc+5
100a-5=5•bc
5bc=100a-5 |:5
bc=20a-1
a=1 => bc=20-1=19 => abc=119
a=2 => bc=40-1=39 => abc=239
a=3 => bc=60-1=59=> abc=359
a=4=> bc=80-1=79 => abc=479
a=5 => bc=100-1=99 => abc=599
abc∈{119; 239; 359; 479; 599}