The average cost of some books is $25.
After 3 new books are added,
it adds another $15 to the total cost.
As a result, the average cost of all the books decreases to $21.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
25 |
21 |
Total value |
25 u |
21 u + 63 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 25 x 1 u
= 25 u
Total cost of books in the end
= 21 x (1 u + 3)
= 21 u + 63
25 u + 15 = 21 u + 63
25 u - 21 u = 63 - 15
4 u = 48
1 u = 48 ÷ 4 = 12
Number of books at first = 12
Answer(s): 12