The average cost of some books is $35.
If 9 of the books are now each $9 more and
3 of the books are now each $6 less,
the average cost of the books becomes $38.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
35 |
38 |
Total value |
35 u |
38 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 35 x 1 u
= 35 u
Increase in the cost after 9 books are now each $9 more and 3 books are now each $6 less
= 9 x 9 - 3 x 6
= 81 - 18
= $63
Total cost of books after 9 books are now each $9 more and 3 books are now each $6 less
= 35 u + 63
Total cost of books in the end
= 38 x 1 u
= 38 u
38 u = 35 u + 63
38 u - 35 u = 63
3 u = 63
1 u = 63 ÷ 3 = 21
Number of books in the end
= 1 u
= 21
Answer(s): 21