The average cost of some books is $45.
If 7 of the books are now each $15 more and
5 of the books are now each $7 less,
the average cost of the books becomes $47.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
45 |
47 |
Total value |
45 u |
47 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 45 x 1 u
= 45 u
Increase in the cost after 7 books are now each $15 more and 5 books are now each $7 less
= 7 x 15 - 5 x 7
= 105 - 35
= $70
Total cost of books after 7 books are now each $15 more and 5 books are now each $7 less
= 45 u + 70
Total cost of books in the end
= 47 x 1 u
= 47 u
47 u = 45 u + 70
47 u - 45 u = 70
2 u = 70
1 u = 70 ÷ 2 = 35
Number of books in the end
= 1 u
= 35
Answer(s): 35