The average cost of some books is $30.
After 5 new books are added,
it adds another $10 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 + 5 |
Value |
30 |
25 |
Total value |
30 u |
25 u + 125 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 30 x 1 u
= 30 u
Total cost of books in the end
= 25 x (1 u + 5)
= 25 u + 125
30 u + 10 = 25 u + 125
30 u - 25 u = 125 - 10
5 u = 115
1 u = 115 ÷ 5 = 23
Number of books at first = 23
Answer(s): 23