Let's assume
number of tenors is x
total number of students =90
2/5 of the singers are sopranos
so, total number of sopranos are

1/3 are altos
so, total number of altos are

we know that
total number of sopranos + total number of altos +total number of tenors = total number of student
we get

now, we can solve for x



Hence ,
number of tenors is 24.............Answer