Diameter of cylindrical tank = 2.8 m

radius = 1.4 m

Height = 4.2 m

Diameter of the pipe = 7 cm

the radius (r) = 7/2 cm

let the length of water in the pipe = h_{1}

h_{1} = 6720 m

Thus, the time taken at the speed of 4 m per second = 6720/(4 x 60) min = 28 min