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Let's review the information given to us to answer the exercise correctly:
Amount in Melissa's prepaid card = $ 30
Price per minute of long distance calls = $ 0.21
Balance of Melissa's prepaid card = $ 21.81
Step 1: Write the equation to find the number of minutes Melissa used on her long distance call, as follows:
Let x to represent the number of of minutes Melissa used on her long distance call
Amount of prepaid card - Number of minutes Melissa talked = Balance of the prepaid card
Replacing with the values we know, we have:
30 - 0.21x = 21.81
Step 2: Solve for x
-0.21x = 21.81 - 30
-.021x = - 8.19
Dividing by -0.21 at both sides:
-0.21x/-0.21 = -8.19/-0.21
x = 39
Step 3: Interpret the result for x
Melissa used her phone card for 39 minutes