In the following question, sentence given with blank to be filled in with an appropriate word(s). Four alternatives are suggested for the question. Choose the correct alternative out of the four and indicate it by selecting the appropriate option.
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?