What is a "Beka"?
Rashi: It is the name of a half-Shekel coin. 1
Which Targum Onkelos translates as 'Tikla' (Shekel) and Targum Yonsan as 'Dark'mon'.
How come that the numbers here (regarding those who donated towards the Mishkan) tallied with the numbers in Bamidbar, 2:32 (regarding the post-Mishkan census), despite the fact that there the Levi'im were not counted, whereas here, they also donated towards the Mishkan?
Sifsei Chachamim: Because also the Levi'im donated freely towards the Mishkan, they were not obligated to give the half-Shekel, which came to atone for the sin of the Eigel, of which they were not guilty.