Let the frequency of 1st tuning fork be x.
Frequency of last tuning fork be x +11y.
8x = x + 11y
7x = 11y ---------------- (1)
Frequency of fifth tuning fork = x + 4y
90 = x + 4y --------------(2)
Using equation (1) and (2), we get:
y = 16.15 and x = 25.38