a) 3/8 < (n + 1)/12
8(n + 1) > 3 x 12
8n + 8 > 36
8n > 24
n > 24:8
n > 3
(n + 1)/ 12 < 5/6
6n + 6 < 5 x 12
6n < 60 -6
6n < 54
n < 9
Deci n ∈(3,9)
b) 15(n - 1) < 12 x 7
15n - 15 < 84
15n < 84 + 15 = 99
n < 99/15 = 33/5 = 6,6
30(n - 1) > 12 x 11 = 132
30n - 30 > 132
30n > 162
n > 162/30 = 5,4
n∈ (5.4, 6.6)
