The average cost of some books is $25.
If 9 of the books are now each $10 more and
2 of the books are now each $6 less,
the average cost of the books becomes $27.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
25 |
27 |
Total value |
25 u |
27 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 9 books are now each $10 more and 2 books are now each $6 less
= 9 x 10 - 2 x 6
= 90 - 12
= $78
Total cost of books after 9 books are now each $10 more and 2 books are now each $6 less
= 25 u + 78
Total cost of books in the end
= 27 x 1 u
= 27 u
27 u = 25 u + 78
27 u - 25 u = 78
2 u = 78
1 u = 78 ÷ 2 = 39
Number of books in the end
= 1 u
= 39
Answer(s): 39