downsample data use fft question

I have some question. For example.

The original signal X has 100000 points, the sample rate is 2kHz；

but the output signal Y has 2000 points, the value of each point is the mean of origin signal per 50 points.

How should I calculate the Fast Fourier Transform? My sample rate becomes 40Hz?

If we don't have original data, do I need to use downsample or upsample?

This is my code:

Fs = 40;

N = length(Y);

n = 2^nextpow2(N);

X_fft = fft(Y,n);

mag = abs(Y_fft);

f = Fs*(0:(N/2))/N;

plot(f,mag(1:N/2+1))

Is that correct

Thanks

[Star Strider]

If you want to downsample your data, use the Signal Processing Toolbox resample (link) function.

[HuanChinag Lin]

my data Y has been downsampled，if I want to analysis the original data X by using data Y，is my code correct?

[Star Strider]

I cannot run your code.

I prefer the calculations in this version of the fft (link) documentation.