The average cost of some books is $20.
After 5 new books are added,
it adds another $20 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 + 5 |
Value |
20 |
15 |
Total value |
20 u |
15 u + 75 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 20 x 1 u
= 20 u
Total cost of books in the end
= 15 x (1 u + 5)
= 15 u + 75
20 u + 20 = 15 u + 75
20 u - 15 u = 75 - 20
5 u = 55
1 u = 55 ÷ 5 = 11
Number of books at first = 11
Answer(s): 11