Boyle's Law for Constant Temperature

Steps:

• Identify initial conditions: pressure $p$, length 20 cm, so initial volume $V_1 \propto 20$ cm.
• Note new length: 20 cm + 40 cm = 60 cm, so new volume $V_2 \propto 60$ cm.
• Apply Boyle's law: $p_1 V_1 = p_2 V_2$ since temperature is constant.
• Solve for new pressure: $p_2 = p \times \frac{20}{60} = \frac{p}{3}$.

Why B is correct:

• Boyle's law states that pressure inversely proportional to volume at constant temperature, so tripling volume divides pressure by 3.

Why the others are wrong:

• A: Halving pressure assumes doubling volume, but volume triples.
• C: Doubling pressure would require halving volume, opposite of expansion.
• D: Tripling pressure would require dividing volume by 3, not expansion.

Final answer: B