Given the following context:

In 1670, Charles II incorporated by royal charter the Hudson's Bay Company (HBC), granting it a monopoly on the fur trade in the area known as Rupert's Land, which would later form a large proportion of the Dominion of Canada. Forts and trading posts established by the HBC were frequently the subject of attacks by the French, who had established their own fur trading colony in adjacent New France.Two years later, the Royal African Company was inaugurated, receiving from King Charles a monopoly of the trade to supply slaves to the British colonies of the Caribbean. From the outset, slavery was the basis of the British Empire in the West Indies. Until the abolition of its slave trade in 1807, Britain was responsible for the transportation of 3.5 million African slaves to the Americas, a third of all slaves transported across the Atlantic. To facilitate this trade, forts were established on the coast of West Africa, such as James Island, Accra and Bunce Island. In the British Caribbean, the percentage of the population of African descent rose from 25% in 1650 to around 80% in 1780, and in the Thirteen Colonies from 10% to 40% over the same period (the majority in the southern colonies). For the slave traders, the trade was extremely profitable, and became a major economic mainstay for such western British cities as Bristol and Liverpool, which formed the third corner of the triangular trade with Africa and the Americas. For the transported, harsh and unhygienic conditions on the slaving ships and poor diets meant that the average mortality rate during the Middle Passage was one in seven.In 1695, the Parliament of Scotland granted a charter to the Company of Scotland, which established a settlement in 1698 on the Isthmus of Panama. Besieged by neighbouring Spanish colonists of New Granada, and afflicted by malaria, the colony was abandoned two years later. The Darien scheme was a financial disaster for Scotland—a quarter of Scottish capital was lost in the enterprise—and ended Scottish hopes of establishing its...

answer the following question:

In what year did the African population in the thirteen colonies contribute to 10%? 

1650