To find the percentage difference between two numbers, we first need to find the absolute difference between them. In this case, the absolute difference between 64 and 85 is 21 (85 - 64 = 21).

Next, we need to find the average of the two numbers. The average of 64 and 85 is (64 + 85) / 2 = 74.5.

Now, we can calculate the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 64 and 85 is (21 / 74.5) x 100 = 28.19%.

This means that the second number (85) is 28.19% higher than the first number (64).