To find the percentage of a data set you use this equation.
x%/100 = is/of
Your problem says 180 of the 600 counters in the bag are red. Work out 180 as a percentage.
So therefore write
x% 180
--- = --------
100% 600
Cross multiply: 1800 * 100 & 600 * x
You should end up with 600x=18,000
Divide 600 by 18,000 to get the variable x by itself: 600x=18000
--------- ---------
600 600
x=30
Therefore, 30% of the 600 counters in the bag are red.