CLAIM: Stephen says that 75 were in Joseph’s family. However, in Genesis 46:27, there are only 70.
RESPONSE: The Septuagint also gives the number at 75 in these passages, and this was the standard Bible that the NT authors used in their writing. Stott writes, “This is the number given in the LXX translation of Genesis 46:27 and Exodus 1:5, although the Hebrew text in both verses has seventy, the discrepancy being probably due to whether Joseph’s sons are included in the total or not.” Both the Septuagint and the Hebrew Bible (MT) are correct on this figure, but it depends on how you count these travelers. The dispute is over whether Joseph’s grandchildren should be counted or not. If these are counted, then the number is 75. If they aren’t counted, then the number is 70.
 Stott, J. R. W. (1994). The message of Acts: The Spirit, the church & the world. The Bible Speaks Today (133). Leicester, England; Downers Grove, IL: InterVarsity Press.