The average cost of some books is $30.
If 8 of the books are now each $15 more and
4 of the books are now each $5 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 8 books are now each $15 more and 4 books are now each $5 less
= 8 x 15 - 4 x 5
= 120 - 20
= $100
Total cost of books after 8 books are now each $15 more and 4 books are now each $5 less
= 30 u + 100
Total cost of books in the end
= 32 x 1 u
= 32 u
32 u = 30 u + 100
32 u - 30 u = 100
2 u = 100
1 u = 100 ÷ 2 = 50
Number of books in the end
= 1 u
= 50
Answer(s): 50