abc:7=bc, rest a => a<7
=> abc=7·bc+a
100·a+bc=7·bc+a
100·a-a=7·bc-bc
99·a=6·bc /:3
33·a=2·bc => a este divizibil cu 2; a∈{2;4;6}
bc=33·a/2
a=2=> bc=33·2/2; bc=33; abc=233
a=4 => bc=33·4/2; bc=66; abc=466
a=6=> bc=33·6/2; bc=99; abc=699
abc∈{233; 466; 699}
