The common difference on the sequence is -3 and the number of books will be 21.
<h3>How to illustrate the sequence?</h3>
The formula that can be used to calculate the number of books will be:
= a + (n - 1)d
= 57 + (13 - 1)-
= 57 + (12 × -3)
= 57 - 36
= 21
Also, from the information given, each row has three fewer books than the row below it. Therefore, the common difference is -3.
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