Abraham Lincoln did his homework, and here is the proof. Pretty interesting the type of math problems they worked through back then; seems much like today. Some math just doesn't change....

Here is one of his problems:

If 100 dollars in one year gain 3½ dollars interest, what sum will gain $38.50 cents in one year and a quarter?

So approach this with what is given:

• ONE YEAR you get 3.5 dollars interest on a deposit amount of $100. So that is 3.5% interest.

What is asked:

• How much did you have to deposit to get $38.5 in ONE AND A QUARTER YEARS?
• The interest rate is still 3.5% PER YEAR

Solution: Lets use ratios

1) The original $100 would give how much in an additional 1/4 of a year? That would be 1/4 of what you get in one year, so 1/4 of 3.5 = 0.875. So the total I get for one year plus a quarter year is 3.5 + 0.875 = $4.375

That means for every $100 I deposit, I get $4.375 for every 1.25 years.

That also means for every $1 I deposit, I get $0.04375. That's 4.375 cents of interest per $1.

2) Since the total interest I want to have is $38.5 then I can divide it by $0.04375 and the result is $880.

** FINAL ANSWER: $38.5 / $0.04375 = $880 **

Lincoln's Solution.

He worked in Pennies not dollars, so he figured for the original $100 he got 350 cents per year. He did this:

350 * 15 months = 5250 cents.

3850 cents * 12 months = 46200 cents

46200 divided by 5250 * 100 = $880

Basically the same thing once you work through it.