A tank used to dampen liquid flow rate surges is known to exhibit second-order dynamics. The input flow rate changes suddenly from 180 to 210 gal/min. An operator notes that the tank level changes as follows:

Before input change: level =10 ft and steady 

Four minutes later: level = 16.5ft 

Forty minutes later: level = 15 ft and steady
(a) Find a transfer function model that describes this process, at least approximately. Evaluate all parameters in your model, including units.
(b) Is your model unique? Why or why not?