The average cost of some books is $25.
If 8 of the books are now each $13 more and
2 of the books are now each $7 less,
the average cost of the books becomes $30.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
25 |
30 |
Total value |
25 u |
30 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 8 books are now each $13 more and 2 books are now each $7 less
= 8 x 13 - 2 x 7
= 104 - 14
= $90
Total cost of books after 8 books are now each $13 more and 2 books are now each $7 less
= 25 u + 90
Total cost of books in the end
= 30 x 1 u
= 30 u
30 u = 25 u + 90
30 u - 25 u = 90
5 u = 90
1 u = 90 ÷ 5 = 18
Number of books in the end
= 1 u
= 18
Answer(s): 18