To find the percentage difference between two numbers, we first need to calculate the absolute difference between them. In this case, the absolute difference between 66 and 99 is 33.

Next, we need to find the average of the two numbers. The average of 66 and 99 is (66+99)/2 = 82.5.

Now, we can calculate the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 66 and 99 is (33/82.5) x 100 = 40%.

Therefore, the percentage difference between 66 and 99 is 40%. This means that 99 is 40% more than 66.