To find the average amount of flour in the bag that changes each day, we start with the total amount of flour used and divide it by the number of days.
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Total amount of flour used: The restaurant uses 15 pounds of flour in 4 days.
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Average amount used per day:
\[ \text{Average amount used per day} = \frac{\text{Total amount of flour}}{\text{Number of days}} = \frac{15 \text{ pounds}}{4 \text{ days}} = 3.75 \text{ pounds per day} \]
So, the average amount of flour in the bag changes by 3.75 pounds each day.
Thus, the answer is 3.75 pounds.