A work is started by a man and it is assumed that he will finish the work in 11 days if working alone. Each subsequent day a new man joined the work. In how many days the four times the original work will be completed, if after the 8th day from the starting of the work no new man will be further added?

One man completes 1/11 of the work per day. On day n, n men work, completing n/11 of the work. After day 8, no new men are added, so 9 men work from day 9 onwards. Sum of work from day 1 to day 8 is the sum of 1/11 to 8/11 = (1+2+...+8)/11 = 36/11. From day 9 onwards, 9 men work, so they complete 9/11 per day. To complete 4 times (44/11) of the work: 36/11 + 9/11 = 45/11, which exceeds 44/11, so the work completes on day 9.