The average cost of some books is $60.
If 7 of the books are now each $11 more and
3 of the books are now each $7 less,
the average cost of the books becomes $62.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
60 |
62 |
Total value |
60 u |
62 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 60 x 1 u
= 60 u
Increase in the cost after 7 books are now each $11 more and 3 books are now each $7 less
= 7 x 11 - 3 x 7
= 77 - 21
= $56
Total cost of books after 7 books are now each $11 more and 3 books are now each $7 less
= 60 u + 56
Total cost of books in the end
= 62 x 1 u
= 62 u
62 u = 60 u + 56
62 u - 60 u = 56
2 u = 56
1 u = 56 ÷ 2 = 28
Number of books in the end
= 1 u
= 28
Answer(s): 28