5th order low pass filter transfer function
A low pass filter is as follows:

$$ \frac{Vout}{Vin}= \frac{1}{\tau*s+1} $$

where \$\tau\$ is equal to \$ RC\$.

Since they are linear in the frequency space they multiply:

$$ \frac{Vout}{Vin}= \frac{1}{\tau_1*s+1}*\frac{1}{\tau_2*s+1}*\frac{1}{\tau_3*s+1}*\frac{1}{\tau_4*s+1}*\frac{1}{\tau_5*s+1}\  .$$

If your tau's are all the same then it would be:
$$ \frac{Vout}{Vin}= \left(  \frac{1}{\tau*s+1} \right)^5\ .$$

Realizing these filters are different, as each RC stage will present a load to the stages after that, which is why we use op amps to isolate the impedance from each stage.