The average cost of some books is $25.
After 4 new books are added,
it adds another $10 to the total cost.
As a result, the average cost of all the books decreases to $19.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 4 |
Value |
25 |
19 |
Total value |
25 u |
19 u + 76 |
Number of books at first = 1 u
Number of books in the end = 1 u + 4
Total cost of books at first
= 25 x 1 u
= 25 u
Total cost of books in the end
= 19 x (1 u + 4)
= 19 u + 76
25 u + 10 = 19 u + 76
25 u - 19 u = 76 - 10
6 u = 66
1 u = 66 ÷ 6 = 11
Number of books at first = 11
Answer(s): 11