The average cost of some books is $21.
After 4 new books are added,
it adds another $12 to the total cost.
As a result, the average cost of all the books decreases to $12.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 4 |
Value |
21 |
12 |
Total value |
21 u |
12 u + 48 |
Number of books at first = 1 u
Number of books in the end = 1 u + 4
Total cost of books at first
= 21 x 1 u
= 21 u
Total cost of books in the end
= 12 x (1 u + 4)
= 12 u + 48
21 u + 12 = 12 u + 48
21 u - 12 u = 48 - 12
9 u = 36
1 u = 36 ÷ 9 = 4
Number of books at first = 4
Answer(s): 4