Answer:
This question is business question so I will answer it from business perspective. The least that I can do is offer her a one year package with an advance of $50. The monthly installment along with the interest that she will pay would be:
Monthly Installment including interest = (Amount Due/12months) + (Outstanding Amount * Interest Rate) ....Eq1
So I assume the interest rate is 5% and as we know the outstanding amount is $150.
By putting the values, we have:
Monthly Installment including interest = ($150/12months) + ($150 * 5%)
= $12.5 + $7.5 = $21 per month
Now the outstanding amount for the second month = $150 - $12.5 = $137.5
Now we will use this new outstanding amount to calculate the monthly installments including the interest by putting the values in the equation 1. Similarly for the next coming months the installments would be calculated.