The shearing stress on each rivet is P/A

$\tau = T\rho / J$

Where:

$T = PRn$

$\rho = R$

$J = \Sigma A\rho^2 = AR^2n$

$\tau = \dfrac{PRn(R)}{AR^2n}$

$\tau = \dfrac{P}{A}$ (*okay!*)

This shows that τ = Tρ/J can be used to find the shearing stress at the center of any rivet.