The average cost of some books is $30.
If 7 of the books are now each $14 more and
5 of the books are now each $4 less,
the average cost of the books becomes $32.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
30 |
32 |
Total value |
30 u |
32 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 $14 more and 5 books are now each $4 less
= 7 x 14 - 5 x 4
= 98 - 20
= $78
Total cost of books after 7 books are now each $14 more and 5 books are now each $4 less
= 30 u + 78
Total cost of books in the end
= 32 x 1 u
= 32 u
32 u = 30 u + 78
32 u - 30 u = 78
2 u = 78
1 u = 78 ÷ 2 = 39
Number of books in the end
= 1 u
= 39
Answer(s): 39