Each month, her daughter paid her $50 and watched as Lisa entered the payment in a ledger and updated the amount owed until the money was paid back in full.
Visualizing: To me, I see this as balancing a checkbook but that is because I might be little old school.
Below is the link that shows how to balance a checkbook:
http://www.ehow.com/video_2202016_balancing-checkbook-bank-statement.html