11. From the "Greek Anthology". Four springs rise from the ground. The first fills the basin in one day, the second in 2 days, the third in 3 days, and the fourth in 4 days. How long will it take for all four springs to fill the basin?
11. If all four sources were open, then in twelve days they would fill $12+6+4+3=25$ pools, therefore they would fill one pool in $\frac{12}{25}$ of a day.