The average cost of some books is $22.
After 6 new books are added,
it adds another $15 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 + 6 |
Value |
22 |
13 |
Total value |
22 u |
13 u + 78 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 22 x 1 u
= 22 u
Total cost of books in the end
= 13 x (1 u + 6)
= 13 u + 78
22 u + 15 = 13 u + 78
22 u - 13 u = 78 - 15
9 u = 63
1 u = 63 ÷ 9 = 7
Number of books at first = 7
Answer(s): 7