The average cost of some books is $60.
If 9 of the books are now each $15 more and
5 of the books are now each $7 less,
the average cost of the books becomes $65.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
60 |
65 |
Total value |
60 u |
65 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 60 x 1 u
= 60 u
Increase in the cost after 9 books are now each $15 more and 5 books are now each $7 less
= 9 x 15 - 5 x 7
= 135 - 35
= $100
Total cost of books after 9 books are now each $15 more and 5 books are now each $7 less
= 60 u + 100
Total cost of books in the end
= 65 x 1 u
= 65 u
65 u = 60 u + 100
65 u - 60 u = 100
5 u = 100
1 u = 100 ÷ 5 = 20
Number of books in the end
= 1 u
= 20
Answer(s): 20