How would you do this question? The answer is listed, but I would like to know how they got that answer. Thank you!

7.2-9. Mixing Time in a Helical Mixer. A helical mixer with an agitator pitch/tank diameter =1.0 and with Dt=1.83m and Da/Dt=0.95 is being used to agitate a viscous fluid having a viscosity of 200000cp and a density of 950kg/m3. The value of N=0.3rev/s. Calculate the mixing time and the power per unit volume. Ans. tT=420s