The average cost of some books is $21.
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 $15.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
21 |
15 |
Total value |
21 u |
15 u + 45 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 21 x 1 u
= 21 u
Total cost of books in the end
= 15 x (1 u + 3)
= 15 u + 45
21 u + 15 = 15 u + 45
21 u - 15 u = 45 - 15
6 u = 30
1 u = 30 ÷ 6 = 5
Number of books at first = 5
Answer(s): 5