For the Rule of 78, for a 12 month period, the last term in the sequence is 12 and the series sums to 78.

Question

For an 18 month period, the last term is and the series sum is .

For a 24 month period, the last term is and the series sum is .

For a 30 month period, the last term is and the series sum is .

Answer 1

for a 18 month period, the last term is 18 and the sum is
1+2+3+...+18 ( ----> sum(n) = n(n+1)/2)

= 18(19)/2
= 171

Do the others the same way.

I recall this rule as something we taught way back in the 60's

Here is a challenge for you:
Phone any bank, mortgage company, credit unit or any money institution to see of they have heard of or if they still use the "Rule of 78".
If not, contact your school board and tell them to remove this topic from the syllabus of the course, and btw, tell them to get an updated textbook.