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]. How many amos did it recede per day?
15 amos / 60 days = .25 amos per day