Based on the question, it is evident that the question is based on Boyle's Law which we will use to find the answer necessary.
Now, Boyle's Equation: P₁V₁ = P₂V₂
V₂ = P₁V₁ ÷ P₂
∴ V₂ = (12.5L × 42.0 cm · Hg) ÷ 75 cm · Hg
⇒ V₂ = 7 L
Thus, the volume when the pressure has increased to 75.0 cm Hg is 7 Litres.
