Probability is defined as the fraction, where the numerator is the number of favorable events (in this case number of large blue t-shirts, which is 20 in the table) and the denominator is the number of ALL possible events (in this example it is the sum of all sold t-shirts, when you sum up all the data in the table, you get total of 262 t-shirts).
So, probability that the next t-shirt (large blue) to be sold is:
P = 20/262 = 0.076335877....
which is approximately 0.08 (when rounding to 2 decimal places).
We obtain the percentage of large blue t-shirt being sold next:
P = 8%
