Let x = number of gigabytes used

Since he gets 8 free gigabytes with his plan, he pays $15 per gigabyte for x - 8 of them

40 + 15(x - 8) ≤ 70

40 + 15x - 120 ≤ 70

15x ≤ 150

x ≤ 10

He can use 10 gigabytes, 8 free ones and up to 2 additional at $15 per gigabyte.