The average cost of some books is $25.
If 7 of the books are now each $9 more and
2 of the books are now each $6 less,
the average cost of the books becomes $28.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
25 |
28 |
Total value |
25 u |
28 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 25 x 1 u
= 25 u
Increase in the cost after 7 books are now each $9 more and 2 books are now each $6 less
= 7 x 9 - 2 x 6
= 63 - 12
= $51
Total cost of books after 7 books are now each $9 more and 2 books are now each $6 less
= 25 u + 51
Total cost of books in the end
= 28 x 1 u
= 28 u
28 u = 25 u + 51
28 u - 25 u = 51
3 u = 51
1 u = 51 ÷ 3 = 17
Number of books in the end
= 1 u
= 17
Answer(s): 17