25:7a Together with their relatives, who were all trained and skillful in the songs of the LORD,

All those men had skills to make good music, as well as their relatives. (EEB)
-or-
These men and their relatives from the tribe of Levi were trained to sing. (English Easy-to-Read Version)

25:7b they numbered 288.

Altogether there were 288 ⌊musicians⌋ .
-or-
There were two hundred eighty-eight (288) ⌊Levites who served as musicians⌋ .

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