The average cost of some books is $55.
If 9 of the books are now each $10 more and
3 of the books are now each $7 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 $10 more and 3 books are now each $7 less
= 9 x 10 - 3 x 7
= 90 - 21
= $69
Total cost of books after 9 books are now each $10 more and 3 books are now each $7 less
= 55 u + 69
Total cost of books in the end
= 58 x 1 u
= 58 u
58 u = 55 u + 69
58 u - 55 u = 69
3 u = 69
1 u = 69 ÷ 3 = 23
Number of books in the end
= 1 u
= 23
Answer(s): 23