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.