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 and B together can finish a work in 30 days. They worked for it for 20 days and then B left the work. The remaining work was done by A alone in 20 days more. In how many days can A alone finish the work?