The average cost of some books is $28.
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 $23.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 5 |
Value |
28 |
23 |
Total value |
28 u |
23 u + 115 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 28 x 1 u
= 28 u
Total cost of books in the end
= 23 x (1 u + 5)
= 23 u + 115
28 u + 10 = 23 u + 115
28 u - 23 u = 115 - 10
5 u = 105
1 u = 105 ÷ 5 = 21
Number of books at first = 21
Answer(s): 21