Consider a particle to be
a single unconnected ball
2 balls together count as a single particle (these are the diatomic particles.)
3 balls together count as a single particle.
Using those definition of particles
count the total number of particles = T
count the total number of diatomic particles = D
Total pressure = 0.810atm x [T/D]
Or you could think about it like
0.810atm / D = the pressure per particle.
pressure per particle x # particles = total pressure.