Answer: 25 mL ;
Explanation:
Note: Boyle's law: P1 * V1 = P1 * V2 ;
pressure decreased by half, or "divided by 2" / as such:
so volume would be divided by 2 ; as such: 50. mL /2 = 25 mL.
The answer is: 25 mL .
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Note: This is assuming that temperature is constant.