Probability as a percent
Divide the favourable count by the total, then multiply by 100 to state the chance as a percent.
A worked example
A bag holds 7 red and 18 blue counters, so the chance of red is 7 ÷ 25 = 0.28, which is 28%.
The mistake people actually make
Divide by the total, not by the count of the other colour. When the first item is not replaced, the second denominator drops by one.