Modelling a maximising problem

Hi, I am completely lost with this problem...

The UN wishes to determine the composition of 1000 identical food parcels, each of which is to be packed into a 20 litre container. For each possible food, its density, dietary value per kilogramme and availability are given below.

Item 1:

Density (kg/litre) 0.1

Dietary value (units/kg): 10

Availability (kg): 2000

Item 2:

Density (kg/litre) 0.2

Dietary value (units/kg): 6

Availability (kg): 3000

Item 3:

Density (kg/litre) 0.25

Dietary value (units/kg): 3

Availability (kg): 10000

Model the problem of maximising the total dietary value of the food parcels as a linear programming problem (assuming that the foods can be packed without any air spaces).

So I am guessing that the objective function will be:

F=1*(x1)+1.2*(x2)+0.9*(x3)

(1=10*01, so that it is units/litre, and so on)

Is that correct? And how can I get the constraints? :-/

Thanks a lot!