Sirul 1,4,7,10 e de forma:
(3·1-2), (3·2-2), (3·3-2), (3·4-2),....
99< 3k-2 < 1000 |+2
101 < 3k < 1002 |:3
101/3 < k < 1002/3
(2+99)/3 < k < (3+999)/3
2/3 + 33 < k < 1+333
2/3 + 33 < k < 334
=> k ∈ {34,35,36,...,333}
333-34 + 1 = 300
=> Sunt 300 de numere de 3 cifre.