A store has a sale on books. The price is $17.55 for one book, $16.70 each for 2 books, $15.85 each for 3 books, and $15 each for 4 books. If this pattern continues, how much per book will it cost to buy 7 books?

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Answer 1 · 2016-11-09T11:29:39

$T_7 = $12.45$

Cost for 7 books will be $$12.45$ per book

The price per book forms an AP

$T_n: 17.55" " 16.70" "15.85" "15" "$ they all differ by $-0.85$

$n:" "1" "2" "3" "4$

The cost if 7 books are bought will be $T_7$

$T_7 = a + d(n-1)$

$T_7 = 17.55-0.85xx6$

$T_7 = $12.45$

Answer 2 · 2016-11-09T11:34:20

$color(green)($12.45)$

Price difference with each additional book purchased is $$0.85$
$color(white)("XXX")(=$17.55-$16.70=$16.70-$15.85=$1585-$15.00)$

The formula for price per books seems to be:
$color(white)("XXX")p(n)=$17.55-(n-1) * $0.85$
where $n$ is the number of books purchased.

$p(7)=$17.55-6 * $0.85 = $17.55 - $5.10 = $12.45$