You took 3 mL of a stock solution of sodium phosphate (25 mM) and mixed it with 12 mL of water to make solution A.

You then took 1 mL of a solution A and mixed it with 1 mL of water to make solution B. 

You then took 1 mL of a solution B and mixed it with 4 mL of water to make solution C.

Calculate the concentration (in mM) of sodium phosphate in each of these solutions.