Concentration problem for lab?

The stock solution of each PCR primer is 100 µM. A 1:10 dilution of this stock was prepared to give the working primer stock used to set-up the PCR reaction. You used 2.5 µl of the working stock in a 50 µl PCR reaction. a) What is the concentration of the working primer stock? b) What is the final concentration of each primer in the PCR reaction? Show all units and calculations.