If 100  ml of a 12  m solution of hcl is diluted to a final volume of 500  ml, what would be the final concentration of the diluted hcl?

Dilution formula:
mv = MV

where one side is concentration × volume BEFORE dilution and the other side is concentration × volume AFTER dilution. 

(100mL) × (12 M) = (500mL) × (X)
(1200 M·mL) = (500mL) × (X)
(1200 M·mL) / (500mL) = X
2.4 M = X