What is a "Beka"?
Rashi: It is the name of a half-Shekel coin. 1
Which Targum Onkelos translates as 'Tikla' (Shekel) and Targum Yonasan as 'Dark'mon'.
How can the number here (of those who donated towards the Mishkan) tally with the number in Bamidbar, 2:32 (the post-Mishkan census)? There, the Levi'im were not counted. Here, also they donated towards the Mishkan!
Sifsei Chachamim: The Levi'im were not obligated to give the half-Shekel for the Mishkan, which came to atone for the sin of the Eigel, of which they were not guilty.
Bearing in mind that the two censuses took place in two different years (in Tishri and Iyar, respectively), how is it possible for their totals to have been equal?
Refer to 30:15:4:1-4 and the notes there..