The average cost of some books is $23.
After 4 new books are added,
it adds another $20 to the total cost.
As a result, the average cost of all the books decreases to $17.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 4 |
Value |
23 |
17 |
Total value |
23 u |
17 u + 68 |
Number of books at first = 1 u
Number of books in the end = 1 u + 4
Total cost of books at first
= 23 x 1 u
= 23 u
Total cost of books in the end
= 17 x (1 u + 4)
= 17 u + 68
23 u + 20 = 17 u + 68
23 u - 17 u = 68 - 20
6 u = 48
1 u = 48 ÷ 6 = 8
Number of books at first = 8
Answer(s): 8