The average cost of some books is $21.
After 6 new books are added,
it adds another $12 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 + 6 |
Value |
21 |
15 |
Total value |
21 u |
15 u + 90 |
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
= 15 x (1 u + 6)
= 15 u + 90
21 u + 12 = 15 u + 90
21 u - 15 u = 90 - 12
6 u = 78
1 u = 78 ÷ 6 = 13
Number of books at first = 13
Answer(s): 13