Additional pressure required : 175 mm
Further explanation
Boyle's Law
At a constant temperature, the gas volume is inversely proportional to the pressure applied
[tex]\rm p_1V_1=p_2.V_2\\\\\dfrac{p_1}{p_2}=\dfrac{V_2}{V_1}[/tex]
V₁=250 ml
P₁=700 mmHg
[tex]\tt V_2=\dfrac{4}{5}\times 250=200~ml[/tex]
[tex]\tt P_2=\dfrac{P_1.V_1}{V_2}\\\\P_2=\dfrac{700\times 250}{200}=875~mmHg[/tex]
Additional pressure :
[tex]\tt 875-700=175~mm[/tex]
