The average cost of some books is $30.
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 $24.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
30 |
24 |
Total value |
30 u |
24 u + 72 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 30 x 1 u
= 30 u
Total cost of books in the end
= 24 x (1 u + 3)
= 24 u + 72
30 u + 12 = 24 u + 72
30 u - 24 u = 72 - 12
6 u = 60
1 u = 60 ÷ 6 = 10
Number of books at first = 10
Answer(s): 10