The average cost of some books is $25.
After 2 new books are added,
it adds another $10 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 + 2 |
Value |
25 |
20 |
Total value |
25 u |
20 u + 40 |
Number of books at first = 1 u
Number of books in the end = 1 u + 2
Total cost of books at first
= 25 x 1 u
= 25 u
Total cost of books in the end
= 20 x (1 u + 2)
= 20 u + 40
25 u + 10 = 20 u + 40
25 u - 20 u = 40 - 10
5 u = 30
1 u = 30 ÷ 5 = 6
Number of books at first = 6
Answer(s): 6