The correct answers to these open questions are the following.
Who used khipu, a system of recording numbers by tying knots on strings?
Answer: Inca.
Whose capital city of Tenochtitlán had a population larger than most European cities?
Answer: Aztecs.
For the Inca, a khipu was a system to communicate information by keeping important records. It was not only used by the Inca but other Andean tribes too. Let's have in mind that the Inca did not have a record system that used any form of alphabet.
In the case of the Aztecs, they found their capital city in the middle of the Texcoco Lake. It was the great city of Tenochtitlan. A huge city very well organized in what today is downtown Mexico City. It had impressive water channels that connected Tenochtitlan with other close towns such as Tlatelolco, Mixcoatl, Tlalpan, and Coyoacán.
