No of stages(S) in direct form FIR filter is given as
S = integer (K * Fs/ $$\triangle$$ f)
Where Fs = Sampling Frequency, $$\triangle$$f = Filter transition band, K = 3(assume)
If x(n) is signal with frequency range 0-2.4 MHz and sampled at $$F_s = 400$$ MHz and it is filtered by

Assumptions:
Passband Frequency LPF(1): 1.8 MHz, Stop band Frequency LPF(1) : 4 MHz
Passband Frequency LPF(2): 1.8 MHz ,Stopband Frequency LPF(2) : 2 MHz
Both filters are having flat passbands and stopbands
Passband attenuation of both filters = 0 dB and stop bandattenuation of both filters is infinity.
Calculate total no. of stages $$S_{LPF1} + S_{LPF2}$$