A venturimeter, having a diameter of $7.5$ cm at the throat an $15$ cm at the enlarged end, is installed in a horizontal pipeline of $15$ cm diameter. The pipe carries an incompressible fluid at a steady rate of $30$ litres per second, The difference of pressure head measured in terms of the moving fluid in between the enlarged and the throat of the venturimeter is observed to be $2.45$ m. Taking the acceleration due to gravity as $9.81 \: m/s^2$, the coefficient of discharge of the venturimeter (correct up to two places of decimal) is ____________