The Rule of 78, as I recall, is a way for banks to front load interest payments.

Assume a one year loan with 12 monthly payments. If you add the numbers 1 through 12 you get 78.

Interest paid in the first month would be 12/78 times the total interest for the year. The second month's interest payment would be 11/78 times the total interest for the year. The last month's interest payment would be 1/78 times the total interest for the year.

Now a 20 year loan would have 240 payments. Using the Rule of 78 the first month's interest payment would be 240/28920 times the total interest for the 20 year period, then 239/28920 etc

This is all from memory. I could be wrong!!!!!!!!