Question

how much sand is needed to fill a pit that has 5m deep and 6m wide and 8m long

Answer 1

In the question you have not mentioned whether you wish to ask about the mass or volume of sand required.

I will give you the volume required and you can calculate the mass based on the density.  

Volume = Length * Breadth * Depth = 5 * 6 * 8 = 240 m3