What did the Inca accomplish?

One of the many things the Inca native Americans created was a way of making math. The Incas made the Quipu. It is not a calculator but a storage device. It was used to keep track of many things like population or other such as food sources. The numbers were represented by knots on a string. For more info go to goolge.com and type in "the Quipu made by the Inca tribe."