How did Ghana, Mali, and Songhai become rich?

Mali:
Mali became rich because they had important trade routes which led into a lot of traders traveling through them and they had to pay taxes on their goods, and the leader controlled trade routes.

Ghana and Songhai:
Ghana and Songhai became rich because salt and gold were the basis of trade. Traders exchanged salt for gold, and some gold was sold to traders and salt was valuable.
Done by:

Tala Salhi and Anoud Huneiti