Given,
The number of total student in the band are 90.
The part of the total student from 8th grade is 5/6.
The number of grade 8th students are,
[tex]\begin{gathered} \text{Number of grade 8th student }=\frac{5}{6}\times90 \\ \text{ = }5\times15 \\ \text{ = 75} \end{gathered}[/tex]The number of students in the band from grade 8th is 75.
Hence, 75 are 8th grader are in jazz band.