Question

Suppose that it takes 10.0 h to drain a container of 5100 m3 of water. What is the "mass flow rate," in kilograms per second, of water from the container?

Answer 1

5100 m^3 of water has a mass of 5.1*10^6 kg. Divide that by 10 hours (expressed as seconds) for the mass flow rate in the units you want.

That's a very large container! More like an Olympic swimming pool.