The average cost of some books is $55.
If 8 of the books are now each $12 more and
4 of the books are now each $4 less,
the average cost of the books becomes $57.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
55 |
57 |
Total value |
55 u |
57 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 55 x 1 u
= 55 u
Increase in the cost after 8 books are now each $12 more and 4 books are now each $4 less
= 8 x 12 - 4 x 4
= 96 - 16
= $80
Total cost of books after 8 books are now each $12 more and 4 books are now each $4 less
= 55 u + 80
Total cost of books in the end
= 57 x 1 u
= 57 u
57 u = 55 u + 80
57 u - 55 u = 80
2 u = 80
1 u = 80 ÷ 2 = 40
Number of books in the end
= 1 u
= 40
Answer(s): 40