Answer:
The total combined cost of 15 books is $77.
Step-by-step explanation:
Here, the total number of books purchased = 15
The cost of books = $7 for 6 books
So, the cost of six books = 6 x ( cost of 1 book) = 6 x ($7) = $42
⇒Out of 15 books, the cost of 6 books = $42 .... (1)
The cost of books = $5 for 4 books
So, the cost offour books = 4 x ( cost of 1 book) = 4 x ($5) = $20
⇒Out of 15 books, the cost of 4 books = $20 .... (2)
Now, the left books are at cost $3 each book.
The number of left books = 15 - ( 6+4) = 15 - 10 = 5
So, the cost of books = $3 for 5 books
So, the cost of five books = 5 x ( cost of 1 book) = 5 x ($3) = $15
⇒Out of 15 books, the cost of 5 books = $15 .... (3)
Now, to get the total price of 15 books add (1) + (2) + (3), we get:
$42 + $20 + $15 = $77
Hence, the total combined cost of 15 books is $77.
