Final answer:
During the Missouri Compromise, keeping an equal number of slave and free states was important to maintain a balance of power in the US Senate, preventing any region from gaining too much influence over slavery legislation. The compromise allowed for the admission of Missouri as a slave state and Maine as a free state, and established a boundary for the future prohibition of slavery within the Louisiana Purchase.
Step-by-step explanation:
During the Missouri Compromise, it was crucial that the number of slave states equal the number of free states to maintain a political balance between the two regions in the US Senate. This balance was important because it prevented either side from having too much legislative power, which could influence futures laws on slavery and its expansion.
The plight between free and slave states rose to prominence when Missouri petitioned to join the Union as a slave state in 1819, which threatened this balance and raised the possibility of disunion and civil war amongst the states.
To address this predicament and maintain the balance, the Missouri Compromise included the entry of Maine as a free state alongside Missouri, a slave state, ensuring equal representation in the Senate. It also set a precedent with the Thomas Proviso, which established a geographical boundary (the 36°30' northern latitude line), marking where slavery was prohibited within the Louisiana Purchase territory.