The average cost of some books is $23.
After 5 new books are added,
it adds another $17 to the total cost.
As a result, the average cost of all the books decreases to $16.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 5 |
Value |
23 |
16 |
Total value |
23 u |
16 u + 80 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 23 x 1 u
= 23 u
Total cost of books in the end
= 16 x (1 u + 5)
= 16 u + 80
23 u + 17 = 16 u + 80
23 u - 16 u = 80 - 17
7 u = 63
1 u = 63 ÷ 7 = 9
Number of books at first = 9
Answer(s): 9