The correct answer is - quadrupled.
The number of African Americans in Michigan grew significantly in the period between 1880 and 1920. Initially there were around 15,000 African Americans in Michigan up until 1880, but in the next 40 years, until the year of 1920, the number of African Americans in Michigan quadrupled, reaching around 60,000.
There's two main reasons for the quick increase in the number of the African American population. The first and most important was the migration from the southern states towards the northern states. The second one was the high birth rate of the African Americans in this period of time.