D : 7 = c r
conform teoremei impartirii cu rest
D = 7 × c + r
c = 2 × r
r < 7 ==> r = 1; 2; 3; 4; 5; 6.
D = 7 × 2 × r + r = 14×r + r = 15 × r
D = 15×1 = 15
D = 15 × 2 = 30
D = 15 × 3 = 45
D = 15 × 4 = 60
D = 15 × 5 = 75
D = 15 × 6 = 90
Numerele naturale nenule sunt:
15; 30; 45; 60; 75; 90 .
