A tube of length 20 m and radius r m carries a steady flow of a liquid whose density and viscosity are 2kgm^−3 and 1kgm^−1s^−1, respectively. The velocity of the fluid at a distance x from the centre of the tube is given as   v(x)=4-k*(x^2)/r   .If the friction force exerted by the liquid on the tube is 60pi N, then what is the value of r?