What is a Concentration Ratio?

-Say that 3 Firms have got Revenues of £100, £80 and £70, and the Market is £500 -You would do the 3 Revenues Firm, Divide by the Market Value and then x100 for a % -Here it is: (£100+£80+£70)/(£500) x 100 = 50% -The 3 Firm Concentration Ratio is 50%