The average cost of some books is $30.
If 7 of the books are now each $15 more and
6 of the books are now each $4 less,
the average cost of the books becomes $33.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
30 |
33 |
Total value |
30 u |
33 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 30 x 1 u
= 30 u
Increase in the cost after 7 books are now each $15 more and 6 books are now each $4 less
= 7 x 15 - 6 x 4
= 105 - 24
= $81
Total cost of books after 7 books are now each $15 more and 6 books are now each $4 less
= 30 u + 81
Total cost of books in the end
= 33 x 1 u
= 33 u
33 u = 30 u + 81
33 u - 30 u = 81
3 u = 81
1 u = 81 ÷ 3 = 27
Number of books in the end
= 1 u
= 27
Answer(s): 27