Ruth 4:20

וְעַמִּינָדָב הוֹלִיד אֶת נַחְשׁוֹן וְנַחְשׁוֹן הוֹלִיד אֶת-שַׂלְמָה

Aminadav begot Nachshon, and Nachshon begot Salma.

The last five verses of Ruth detail the generations from Peretz to David. The Ibn Ezra wants to calculate how many years it was from the Exodus to the birth of David. If we assume that Nachshon was under the age of 20 when he left Egypt, he could have entered Israel. If so, that would make it 426 years, which is an average of 84 years and change for each of five generations. This may seem like a lot but we know that Boaz was already old when he married Ruth (3:10) and that David was the youngest of eight children, so his father Jesse must have been older, as well.