Answer
142 mL
Work Step by Step
Using the equation for molarity:
$M_{i}V_{i}= M_{f}V_{f}$
$\implies V_{i}=\frac{M_{f}V_{f}}{M_{i}}=\frac{0.425\,M\times250.0\,mL}{0.750\,M}$
$= 142\,mL$
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