During the flood the water was 15 amos (cubits) over the highest mountain [Bereishis (Genesis) 7:20]. It took 60 days for the water to recede enough to expose the mountaintop [Bereishis 8:5]. The water receded evenly each day [Rashi on Bereishis 8:4]. The Teva (Ark) touched the mountaintop after only 16 days. How deep below the water was the bottom of the Teva?
15 amos / 60 days = .25 amos per day (The water receded at the rate of .25 amos per day)
16 days x .25 amos per day = 4 amos (The water only receded 4 amos before the Teva hit the mountain)
15 amos – 4 amos = 11 amos (The Teva had to sink only 4 amos of the 15 amos in order to hit the mountain, so it was 11 amos deep below the water)