The Pfizer Inc of the United States of America (USA) and its partner BioNTech of Germany completed the phase 3 trial of their COVID-19 vaccine candidate, BNT162b2 against SARS-CoV-2 with 95% effective success rate.
A mixture contains acid and water in the ratio of 7 : 3. If 4 litres of water is added to it, then the ratio of acid and water becomes 7 : 4. What is the quantity (in litres) of acid in the mixture?