Numbers 3

"And you shall take the Levites for Me—I am the Lord—instead of all the firstborn among the children of Israel, and the livestock of the Levites instead of all the firstborn among the livestock of the children of Israel.”  So Moses numbered all the firstborn among the children of Israel, as the Lord commanded him.  And all the firstborn males, according to the number of names from a month old and above, of those who were numbered of them, were twenty-two thousand two hundred and seventy-three." vv.41-43

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In the last chapter we saw all the tribes of Israel counted except the Levites.  Now in this chapter we see why.  They were to be priests and the offering to the Lord instead of the firstborn of every family.   They would serve in the Tabernacle and to the High Priests, Aaron and his sons.  When we look back as to birth order of the twelve sons of Jacob (Israel) we know that Levi wasn't the first born but the third born of his wife Leah.  This could possibly be the fulfillment of Genesis 49 when it is said they will be scattered among Israel.  Also from Exodus 32 at the worship of the golden calf the Levites were the only ones that didn't seem to worship it and obeyed Moses. 

"So the sons of Levi did according to the word of Moses. And about three thousand men of the people fell that day. Then Moses said, 'Consecrate yourselves today to the Lord, that He may bestow on you a blessing this day, for every man has opposed his son and his brother.'" Ex.32:28-29

Did you see that when all the tribes were numbered that those numbered were twenty years old and up, whereas when the tribe of Levi was number it was those one month old and above.   We see another fun mathematician in scripture here:  As the Lord had Moses number all the first borns of the tribes of Israel in verse 39 we see there were 22,000.  Then we see the total number of Levites who were to replace and redeem those of the firstborns as 22,273 (v.43).  So we get the difference of 273.  The Lord doesn't just say "close enough" but has the difference be paid out to Aaron and his sons (v.48).