Now, let's start with the Guitarists: they are 5/8 of the 64 musicians, that is
64*

now, some number to 40 (total number of the guitarists) has the ratio of 1:4
ratios remain equivalent if we multiply both numbers by the same number.
so the ratio of 1:4 is equivalent to 10:40.
this means that there are 10 Jazz solos guitarists.
And how many classical solo guitarists were there? the rest! 40-10=30
so the answer is 30