need help calculating stock primer concentration - (Nov/05/2015 )

I need help back calculating the stock primer concentration with the following information:

There is 0.5uL of a four primer mix (primer 1,2,3,4) added into a PCR mastermix with a final PCR volume of 25ul. The final primer concentration in the PCR mastermix is 25uM each. What is the stock primer concentration of each of the 4 primers?

Well, the concentration would be the same for adding 1 ul of primer mix to a 50 ul reaction. You dilution of the primer mix in the PCR reaction is 50x. Since there are four primers, the dilution of each primer is 200x. The initial primer concentration then is 200 times 25 uM, or 5000 uM, or 5 mM.