abcd barat = 1000a + 100b + 10c + d =
= 100(10a + b) + (10c + d) =
= 100 × ab (barat) + cd (barat) [dar cd(barat) = 8 × ab (barat)]
⇒ 100 × ab (barat) + cd (barat) =
= 100 × ab (barat) + 8 × ab (barat) (Dam factor comun pe ab (barat))
⇒ 100 × ab (barat) + 8 × ab (barat) =
= ab (barat) (100 + 8) =
= 108 × ab (barat) = 27 × 4 × ab (barat) divizibil cu 27