A tank has a hole at its bottom. The time needed to empty the tank from level h1 to h2 will be proportional to

Let -dhdt represent the rate of descent of water level. Let A and a represent the cross-sectional areas of the container and hole respectivelyThen, -Adhdt = a2ghor dt = -kdhndtor ∫0tdt  = -k∫h1h21hdhor t = -k|h12+1-12+1|h2h1or t = -2k[h2 - h1]t is directly proportioanl to  (h1- h2)