The average cost of some books is $23.
After 4 new books are added,
it adds another $17 to the total cost.
As a result, the average cost of all the books decreases to $18.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 4 |
Value |
23 |
18 |
Total value |
23 u |
18 u + 72 |
Number of books at first = 1 u
Number of books in the end = 1 u + 4
Total cost of books at first
= 23 x 1 u
= 23 u
Total cost of books in the end
= 18 x (1 u + 4)
= 18 u + 72
23 u + 17 = 18 u + 72
23 u - 18 u = 72 - 17
5 u = 55
1 u = 55 ÷ 5 = 11
Number of books at first = 11
Answer(s): 11