Answer
$1.92$
Work Step by Step
We know that
$\frac{\Delta P_1\pi r_1^4}{8\eta L}=\frac{\Delta P_2\pi r_2^4}{8\eta L}$
This simplifies to:
$\frac{\Delta P_2}{\Delta P_1}=\frac{r_1^4}{r_2^4}$
$\implies \frac{\Delta P_2}{\Delta P_1}=\frac{r_1^4}{(0.85r_1)^4}=1.92$
