The average cost of some books is $28.
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 $18.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 5 |
Value |
28 |
18 |
Total value |
28 u |
18 u + 90 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 28 x 1 u
= 28 u
Total cost of books in the end
= 18 x (1 u + 5)
= 18 u + 90
28 u + 10 = 18 u + 90
28 u - 18 u = 90 - 10
10 u = 80
1 u = 80 ÷ 10 = 8
Number of books at first = 8
Answer(s): 8