The average cost of some books is $21.
After 6 new books are added,
it adds another $10 to the total cost.
As a result, the average cost of all the books decreases to $17.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 6 |
Value |
21 |
17 |
Total value |
21 u |
17 u + 102 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 21 x 1 u
= 21 u
Total cost of books in the end
= 17 x (1 u + 6)
= 17 u + 102
21 u + 10 = 17 u + 102
21 u - 17 u = 102 - 10
4 u = 92
1 u = 92 ÷ 4 = 23
Number of books at first = 23
Answer(s): 23