Boyle's Law: Pressure inversely proportional to volume at constant temperature

Steps:

• Initial volume proportional to length: V₁ ∝ 20 cm.
• Final length is 20 cm + 40 cm = 60 cm, so V₂ ∝ 60 cm.
• By Boyle's Law (PV = constant), p × V₁ = P₂ × V₂.
• Solve for P₂: P₂ = p × (20/60) = p/3.

Why B is correct:

• Boyle's Law states that for ideal gas at constant temperature, pressure decreases inversely with volume increase; here volume triples, so pressure becomes one-third.

Why the others are wrong:

• A: Assumes volume doubles (pressure halves), but it triples.
• C: Assumes volume halves (pressure doubles), opposite of expansion.
• D: Assumes volume one-third (pressure triples), but expansion reduces pressure.

Final answer: B