The average cost of some books is $30.
After 2 new books are added,
it adds another $20 to the total cost.
As a result, the average cost of all the books decreases to $25.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 2 |
Value |
30 |
25 |
Total value |
30 u |
25 u + 50 |
Number of books at first = 1 u
Number of books in the end = 1 u + 2
Total cost of books at first
= 30 x 1 u
= 30 u
Total cost of books in the end
= 25 x (1 u + 2)
= 25 u + 50
30 u + 20 = 25 u + 50
30 u - 25 u = 50 - 20
5 u = 30
1 u = 30 ÷ 5 = 6
Number of books at first = 6
Answer(s): 6