The average cost of some books is $24.
After 3 new books are added,
it adds another $20 to the total cost.
As a result, the average cost of all the books decreases to $20.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
24 |
20 |
Total value |
24 u |
20 u + 60 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 24 x 1 u
= 24 u
Total cost of books in the end
= 20 x (1 u + 3)
= 20 u + 60
24 u + 20 = 20 u + 60
24 u - 20 u = 60 - 20
4 u = 40
1 u = 40 ÷ 4 = 10
Number of books at first = 10
Answer(s): 10