The average cost of some books is $25.
If 6 of the books are now each $15 more and
5 of the books are now each $4 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 6 books are now each $15 more and 5 books are now each $4 less
= 6 x 15 - 5 x 4
= 90 - 20
= $70
Total cost of books after 6 books are now each $15 more and 5 books are now each $4 less
= 25 u + 70
Total cost of books in the end
= 27 x 1 u
= 27 u
27 u = 25 u + 70
27 u - 25 u = 70
2 u = 70
1 u = 70 ÷ 2 = 35
Number of books in the end
= 1 u
= 35
Answer(s): 35