The average cost of some books is $28.
After 3 new books are added,
it adds another $12 to the total cost.
As a result, the average cost of all the books decreases to $22.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
28 |
22 |
Total value |
28 u |
22 u + 66 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 28 x 1 u
= 28 u
Total cost of books in the end
= 22 x (1 u + 3)
= 22 u + 66
28 u + 12 = 22 u + 66
28 u - 22 u = 66 - 12
6 u = 54
1 u = 54 ÷ 6 = 9
Number of books at first = 9
Answer(s): 9