Ram, Ravi and Ratan can alone finish an assignment in 9 days, 12 days and 15 days respectively. They decide to complete a work by working in turns. Ram works alone on Monday, Ravi does the work alone on Tuesday, followed by Ratan working alone on Wednesday & so on. What proportion of the complete work is done by Ravi?

Part of the work  completed on Monday, Tuesday and Wednesday = 1/9 +1/12 + 1/15 = 47/180

Part of the work completed at the end of 9 days of work = 47(3)/180 = 47/60

remaining part of the work= 13/60 = 39/180

Part of the work completed by ram on 10th day = 1/9 = 20 /180

Part of the work completed by Rvi on 11th day = 1/12 = 15/180

remaining part of the work = 4/180

Ratan can do 12//180th of work on 12th day. Hence he needs 1/3rd of the 12th day.

Proportion of the completed work done by Ravi = 3(1/12) + 15/180 = 1/3