The average cost of some books is $20.
After 3 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 + 3 |
Value |
20 |
15 |
Total value |
20 u |
15 u + 45 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 20 x 1 u
= 20 u
Total cost of books in the end
= 15 x (1 u + 3)
= 15 u + 45
20 u + 15 = 15 u + 45
20 u - 15 u = 45 - 15
5 u = 30
1 u = 30 ÷ 5 = 6
Number of books at first = 6
Answer(s): 6