A mathematical mean is the average of a group of numbers.
The mean daily decrease would be the total amount of decrease (in this case, $25) divided by the total amount of days it took to get there (in this case, 4).
25 / 4 = 6.25 = 6 \(\frac{1}{4}\)
The mean daily decrease in price would be 6 \(\frac{1}{4}\).
Hope that helped! =)
