Related do Liczb 8:24

A Levite may only enter the court for service if he has studied for five years, for it is said: "This applies to the Levites: from the age of twenty-five" etc. And further the [Scripture] says: ”From the age of thirty”. If it is said: “From the age of twenty-five” why is it said: ״From thirty years”? If it is said: "From the age of thirty" then why is it said: "From the age of twenty-five"? That means that he studied all the time between the age of twenty-five and thirty, from that point in time and onwards he was allowed to work From here it was said: Whoever does not show success in his instruction within five years, R. Jose says three years, because it is said: "To educate you for three years.