One type of capillary$-$tube viscometer is shown in figure below. For this device, the liquid to be tested is drawn into the tube to a level above the top etched line. The time is then obtained for the liquid to drain to the bottom etched line. The kinematic viscosity, u $,$ in m$2/$s$,$ is then obtained from the equation u $=$ KR$4$ t$,$ where K is a constant, R is the radius of the capillary tube in mm$,$ and t is the drain time in seconds. When glycerin at $20\backslash$deg C is used as a calibration fluid in a particular viscometer, the drain time is $1430$ s$.$ When a liquid having a density of $970$ kg$/$m$3$ is tested in the same viscometer, the drain time is $900$ s$.$ What is the dynamic viscosity of this liquid?