Decade counter is a synchronous term for BCD counter that counts the BCD digits from 0 - 9 and hence the mod value of decade counter = 10
Hence the frequency will be divided by 10 if we have a decade counter.
Given input frequency = 640 KHz
After going through 4 bit ring counter , we have = 640 / 4 = 160 KHz(As mod value of n bit ring counter is n)
After going through 4 bit Johnson counter we have = 160 / 8 = 20 KHz(As mod value of n bit Johnson counter is 2n)
After going through the decade counter finally, we have = 20 / 10 = 2 KHz
Thus the final output frequency = 2 KHz