**Part (b):** We have to find the diameter of the nozzle if the speed of outcoming water is 21 m/s. If ‘r’ is the radius of the nozzle, then area of the cross-section of the nozzle will be A_{2} = πr^{2}. For convenience, we put area of the pipe as A_{1} = π r_{1}^{2}, where r_{1} = 0.048 m = radius of the cross-section of the pipe. Therefore,

Since diameter is 2 times the radius, therefore,

This is the answer.