math question

9.75x=13/16

x=1/12 

1/12 each hour

between 10pm and 7:45am water level decreased by 13/16 how much did the water drop each hour

Number of hours between 10pm and 7:45am =9 3/4 hours

(13/16) / 9 3/4 =1/12, or about 8.33% drop in water level per hour.

.....  8.33%   of AN INCH.