re the SG query
1 cubic metre of H2O weighs 1000kg. By definition the SG=1 (it's water...)_
If you have a different substance that weighs 780kg/m^3 then it must be
780
-------- = .78 SG
1000
In other words, they gave you the fundamentals of SG & expect you to to know enough about SG to recognise it when presented in a different form.