Answer
There is a 68 percent reduction in the blood flow rate,
Work Step by Step
Use Poiseuille’s equation. The volume flow rate is proportional to $R^4$, all else being equal. In other words, $\frac{Q}{R^4}$ remains constant. Note that when the diameter is reduced by 25 percent, so is the radius.
$$\frac{Q_{final}}{R^4_{final}}=\frac{Q_{initial}}{R^4_{initial}}$$
$$\frac{Q_{final}}{ Q_{initial}}=\frac{ R^4_{final}}{R^4_{initial}}=(0.75)^4=0.32$$
The final flow rate is 32 percent of the original, which is a reduction of 68 percent.