Concentration of all species

You have the following hypothetical decomposition:
A3BC2D4 ----> 3A + B + 2C + 4D
If you have 0.0010 moles of D, how much of A, B, and C do you have? It's been so long since I've taken chemistry...

The easiest way to do this is to first find a ratio between a species that has stoichiometric ratio of 1 (in this case, B). The mass balance has the general formula of:

(1/a)A = (1/b)B

Where a and b are the stoichiometry numbers and A and B are the species (in a generic sense, not the example given). From above, B has a stoichiometric number of 1 and D has a stoichiometric number of 4, so:

B = (1/4)D

Solving gives:

B = (1/4)*0.001 = 0.00025 moles

Now you can solve for the rest:

B = (1/3)A; A = 0.00075 moles.

B = (1/2)C; C = 0.00050 moles.

So:
A = 0.00075 moles
B = 0.00025 moles
C = 0.00050 moles
D = 0.0010 moles

Without volume, can't really find concentration, but if you have it, just divide the moles by volume.