Final answer:
Melissa's call lasted for 39 minutes. This was calculated by subtracting her remaining credit from the initial amount, then dividing the result by the cost per minute.
Step-by-step explanation:
To determine how many minutes the long-distance call that Melissa made lasted, we need to calculate the total amount of money spent on the call. We do this by subtracting the remaining credit on her card from the original amount she purchased the card for. So, $30.00 - $21.81 = $8.19 spent on the call. Knowing that long-distance calls cost 21 cents a minute, we divide $8.19 by $0.21 to get the number of minutes.
$8.19 ÷ $0.21 = 39 minutes. Melissa's prepaid phone card had $30.00 and now has $21.81 left, so she used $8.19 worth of minutes. At 21 cents per minute, she used her card for 39 minutes.