Suppose the marginal cost of the 1st hour of talking on the phone is $50, the marginal cost of the 2nd hour is $75, and the marginal cost of the 3rd hour is $105. In this case, the total cost of talking on the phone for 3 hours is:

$230

Explanation:

Data given in the question

Marginal cost of the first hour = $50

Marginal cost of the second hour = $75

Marginal cost of the third hour = $105

So by considering the above information, the total cost is

= Marginal cost of the first hour + Marginal cost of the second hour + Marginal cost of the third hour

= $50 + $75 + $105

= $230

We simply added the marginal cost of all three hours in order to determine the total cost

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