Application of linear inequalities

Question

For his mobile phone service, John pays a monthly fee of $40 plus $15 for each 1 GB of data (rounded up) over his 8 GB of data that comes with his plan. For his monthly charge before taxes to be at most $70, how many gigabytes of data can he use in a month?

Answer 1

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.