Answer:
$947
Step-by-step explanation:
Amount of money Erin owed to Katie on Jan 1, 2009 = $17,605
owed to Katie on Jan 1, 2010 = $6,241
Amount of money Erin paid Katie each month during 2009 = $17,605 - $6,241 = $11,364
Average amount of Erin monthly payment = $11,364 / 12 months = $947