The inequality that will determine the number of months, x, that are required for the second phone to be less expensive is 100 + 55x > 150 + 51x
.

The solution to the inequality is
.

Sal’s mother would have to keep the second cell phone plan for at least
months in order for it to be less expensive.

a) The first inequality 100+55x>150+51x;

b) The last inequality x>12.5

c) 13 months

Step-by-step explanation:

a) Let x be the number of months.

1. The first phone costs $100 and $55 per month for unlimited usage, then for x months it will cost $55x and in total

$(100+55x)

2. The second phone costs $150 and $51 per month for unlimited usage, then for x months it will cost %51x and in total

$(150+51x)

3. If the second phone must be less expensive than the first phone, then

150+51x<100+55x

or

100+55x>150+51x

b) Solve this inequality:

55x-51x>150-100

4x>50

x>12.5

c) Sal's mother has to keep the second cell phone for at least 13 months (because x>12.5).

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