$20 is the x factor which is independent, and the teens demand is the y factor which is dependent.
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https://socratic.org/questions/how-were-the-west-african-kingdoms-involved-in-the-slave-trade
With the development of the trans-Saharan slave trade and the economies of gold in the western Sahel, a number of the major states became organized around the slave trade, including the Ghana Empire, the Mali Empire, and Songhai Empire. However, other communities in West Africa largely resisted the slave trade.
The transatlantic slave trade began during the 15th century when Portugal, and subsequently other European kingdoms, were finally able to expand overseas and reach Africa. The Portuguese first began to kidnap people from the west coast of Africa and to take those they enslaved back to Europe
Persians and Byzantines were already weak from fighting among themselves. <span />
