The average cost of some books is $23.
After 5 new books are added,
it adds another $19 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 + 5 |
Value |
23 |
15 |
Total value |
23 u |
15 u + 75 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 23 x 1 u
= 23 u
Total cost of books in the end
= 15 x (1 u + 5)
= 15 u + 75
23 u + 19 = 15 u + 75
23 u - 15 u = 75 - 19
8 u = 56
1 u = 56 ÷ 8 = 7
Number of books at first = 7
Answer(s): 7