\(\because\) Rate of flow will be same across all pipes
So, pressure across the pipe \(\propto \frac{\text { length }}{(\text { radius })^4}\) \(\quad \quad\{\) rate of flow of liquid (Q) \(Q=\frac{\pi P r^4}{8 \eta L}\)
\(\frac{P_1}{P_2}=\frac{\left(L / r^4\right)}{\left(\frac{L / 2}{(r / 2)^4}\right)}=\frac{1}{8}\)
Then \(P_2=8 P_1\)