What size tank (in litres) is needed for 1000kg of (a) water (b) gasoline? (c) mercury?
I know that the Dm for each is (a) 1000kg/m^3 (b) 680kg/m^3 (c) 13,600kg/m^3, but I'm at a loss from here. Please help.

I'll do gasoline for you, then you have to do the rest.

1000kg of gasoline takes up 1000/680 = ~1.47 m^3. there are 1000 liters in 1m^3, so you need a ~1470 liter tank.