Fifteen, Fifteenth - Vine's Expository Dictionary of New Testament Words
Fifteen, Fifteenth[ 1,,G1178, dekapente ]
lit., ten-five," occurs in John 11:18; Acts 27:28; Galatians 1:18.
(1) In Acts 7:14, "threescore and fifteen" translates a different numeral, lit., "seventy-five." This refers to all Joseph's kindred whom he sent for. There is no discrepancy between this and Genesis 46:26. The Sept. translations give the number as 75 in Genesis 46:27 and in Exodus 1:5, and this Stephen follows, being a Grecian Jew.
(2) The corresponding ordinal numeral pentekaidekatos, "fifteenth" (lit., "five and tenth") is found In Luke 3:1, where Luke dates the reign of Tiberias from the period of his joint rule with Augustus.