Answer
The average pressure is $\frac{p}{8}$.
Work Step by Step
Because the second surface is twice as far away as the first surface, the intensity there is reduced by a factor of 4, due to the inverse square law.
Equations 32.32 and 32.33 show that the pressure is proportional to the wave intensity.
Furthermore, because the second surface is perfectly absorbing instead of perfectly reflecting as the first surface is, the pressure there is reduced by another factor of two, as shown by equations 32.32 and 32.33.
The net result is a reduction in pressure by a factor of 8.