The average cost of some books is $29.
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 $23.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 4 |
Value |
29 |
23 |
Total value |
29 u |
23 u + 92 |
Number of books at first = 1 u
Number of books in the end = 1 u + 4
Total cost of books at first
= 29 x 1 u
= 29 u
Total cost of books in the end
= 23 x (1 u + 4)
= 23 u + 92
29 u + 20 = 23 u + 92
29 u - 23 u = 92 - 20
6 u = 72
1 u = 72 ÷ 6 = 12
Number of books at first = 12
Answer(s): 12