My teacher told me it came from the belief that the black man worked 3/5ths of what a whiteman did but I'm not sure if this is accurate, would be great if someone could verify it
From what I’ve seen the number itself is quite arbitrary and rather just comes from southerners wanting slaves to count as 1 full person in the population count. While the northerners did not want them to count at all. So the number was just sort of meeting in the middle I’m unable to find a external reason.