The average cost of some books is $55.
If 9 of the books are now each $9 more and
3 of the books are now each $8 less,
the average cost of the books becomes $58.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
55 |
58 |
Total value |
55 u |
58 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 9 books are now each $9 more and 3 books are now each $8 less
= 9 x 9 - 3 x 8
= 81 - 24
= $57
Total cost of books after 9 books are now each $9 more and 3 books are now each $8 less
= 55 u + 57
Total cost of books in the end
= 58 x 1 u
= 58 u
58 u = 55 u + 57
58 u - 55 u = 57
3 u = 57
1 u = 57 ÷ 3 = 19
Number of books in the end
= 1 u
= 19
Answer(s): 19