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The equation is , where y is the monthly charge and x is the month in which it is calculated.

Step-by-step explanation:

Step 1:

First, we need to establish a relation between the difference in costs. For the first month it cost $72.99, during the second month, it costs $101.44 and in the third month, it costs $129.89. Difference in rates

Second months rate - First months rate =

Third months rate - Second months rate =

So for every consecutive month, the charges increase by $28.45

Step 2:

Now we develop an equation,

, where y is the charge that month and x is the month in which it is calculated.

For x = 1,

For x = 2,

For x = 3,

So this is the equation to calculate the cost y the mobile provider charges in month x.

step-by-step explanation:

(3rd option) - c