To find the average amount of flour used per day, you can use the following formula:
\[ \text{Average amount of flour used per day} = \frac{\text{Total amount of flour}}{\text{Number of days}} \]
In this case, the total amount of flour is 15 pounds, and it is consumed over 4 days:
\[ \text{Average amount of flour used per day} = \frac{15 \text{ pounds}}{4 \text{ days}} = 3.75 \text{ pounds per day} \]
Thus, the average amount of flour in the bag changes by 3.75 pounds each day (rounded to the nearest hundredth, it remains 3.75 pounds).