A section of blood vessel has a flow-rate F in cubic millimetres per

minute proportional to the fourth power of its radius, R. The blood vessel can be considered perfectly cylindrical and is expandable. At some instant the

flow rate is 30 cubic millimetres per minute when the radius is 0.5 millimetres. At that same instant the flow-rate is increasing at rate of 0.24 cubic millimetres per minute squared.

(i) What is the rate of increase of the radius at that instant?

(ii) If the rate of increase of the radius is constant how long after the time when the radius is 0.5 mm is the radius 1 mm?