The average cost of some books is $30.
After 5 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 + 5 |
Value |
30 |
20 |
Total value |
30 u |
20 u + 100 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 30 x 1 u
= 30 u
Total cost of books in the end
= 20 x (1 u + 5)
= 20 u + 100
30 u + 10 = 20 u + 100
30 u - 20 u = 100 - 10
10 u = 90
1 u = 90 ÷ 10 = 9
Number of books at first = 9
Answer(s): 9