Calculation of number of Uranium-235 nuclei

In a problem, I'm supposed to calculate the number of Uranium-235 nuclei in 10^4 Kg of Uranium. The answer book gives Number of U-235 nuclei = (10^4)*(1000/238)(6*10^23)(0.02) = 5*10^26 I don't know where the factor 6*10^23 and (1000/238) come from. The 0.02 come from the fact that 2% of Uranium is U-235 and the rest is U-238. 10^4 is the mass. Is the number 238 somehow associated with U-238? Could you please explain, why this calculation includes (1000/238) and 6*10^23? Thank you.

Speed of tidal waves

Show that the speed of tidal waves is given by (gh)^(1/2) where h is the depth of the sea and g is the acceleration due to gravity.

Power of a propeller turbine

Estimate the power available in a tidal stream flowing at 3m/s using a 5m diameter propeller turbine.