We are draining a cylinder through a single small hole, like our capillary. the capillary is at the height of the bottom of the tube, and the flow is small enough that we can ignore the velocity part of bernoulli, and assume that flow is determined just by pressure and the viscous flow through the capillary. (in simpler words, assume the cylinder draining is exponential.) if it takes 20 seconds for the tube to drop to half its original height, how long does it take to empty 90% of the water from the tube?