The average cost of some books is $21.
After 5 new books are added,
it adds another $17 to the total cost.
As a result, the average cost of all the books decreases to $13.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 5 |
Value |
21 |
13 |
Total value |
21 u |
13 u + 65 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 21 x 1 u
= 21 u
Total cost of books in the end
= 13 x (1 u + 5)
= 13 u + 65
21 u + 17 = 13 u + 65
21 u - 13 u = 65 - 17
8 u = 48
1 u = 48 ÷ 8 = 6
Number of books at first = 6
Answer(s): 6