The average cost of some books is $29.
After 3 new books are added,
it adds another $11 to the total cost.
As a result, the average cost of all the books decreases to $25.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
29 |
25 |
Total value |
29 u |
25 u + 75 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 29 x 1 u
= 29 u
Total cost of books in the end
= 25 x (1 u + 3)
= 25 u + 75
29 u + 11 = 25 u + 75
29 u - 25 u = 75 - 11
4 u = 64
1 u = 64 ÷ 4 = 16
Number of books at first = 16
Answer(s): 16