Picture of cuneiform tablet from the second millennium B.C.

Tablets like this one from the second millennium B.C. in ancient Mesopotamia record a number of textbook interest rate problems, and their solutions. The questions read like those from a high-school algebra book. Sometimes the method at arriving at the solution is just as mysterious. The tablet called VAT 8528 asks:

If I lent one mina of silver at the rate of 12 shekels (1/60 of a mina) per year , and I received in repayment, one talent (60 minas) and 4 minas. How long did the money accumulate?

The Babylonians compounded interest by capitalizing interest when the outstanding principal doubled. At 20% the principal doubled every five years. Thus, it would take 30 years for the debt to grow to 64 minae.