In treble clef, a quarter note is written at B-flat—on the middle line, preceded by a flat accidental.
The letter name of the pitch that is a fifth above the following note is
F.
To determine the pitch that is a fifth above the note B-flat in treble clef, we need to understand the musical interval of a fifth. A fifth is made up of five letter names in the alphabet. Starting from B, the notes would be B, C, D, E, and F.
Since we know that the quarter note is written at B-flat on the middle line, we need to find the note that is a fifth above B-flat. Counting up five notes from B-flat, we get B-flat, B, C, D, and E.
Therefore, the letter name of the pitch that is a fifth above B-flat is E.