a) 6 < a < 9; => a ∈ (7, 8)
б) 315 ≤ t ≤ 317; => t ∈ (315, 316, 317)
в) 16 < x < 20; => x ∈ (17, 18, 19)
г) 108 < n < 112; => n ∈ (109, 110, 111)