Rizvi and Shafi take a project to work together. However, Shafi falls sick after working for 20 days and Rizvi works alone on the project for the last 6 days. They submit the project in 26 days. Had Rizvi and Shafi worked on the project for all the days together, they could have submitted it in 24 days. How many days will Shafi take to complete the project alone?

Let work is done by Rizvi in 1 day = 1/R & Shafi in 1/S day
Both complete work in 24 days. So in 1 day, together they complete = 1/24 = 1/R + 1/S
For 20 days both work together, so work done by them = 20(1/R + 1/S) = 5/6
Remaining work = {1 - (5/6)= 1/6} is done by Rizvi alone in 6 days
Work done by Rizvi in 6 days = 1/6 = 6(1/R)
∴ R = 36 = days needed by Rizvi to complete the work alone

According to the question,
∴ (1/36) + 1/S = 1/24
1/S = (1/24) - (1/36)
1/S = (3 - 2)/72
1/S = 1/72
S = 72 days needed by Shafi to complete the work alone.