The average cost of some books is $20.
After 6 new books are added,
it adds another $15 to the total cost.
As a result, the average cost of all the books decreases to $15.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 6 |
Value |
20 |
15 |
Total value |
20 u |
15 u + 90 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 20 x 1 u
= 20 u
Total cost of books in the end
= 15 x (1 u + 6)
= 15 u + 90
20 u + 15 = 15 u + 90
20 u - 15 u = 90 - 15
5 u = 75
1 u = 75 ÷ 5 = 15
Number of books at first = 15
Answer(s): 15