The average cost of some books is $23.
After 6 new books are added,
it adds another $13 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 + 6 |
Value |
23 |
18 |
Total value |
23 u |
18 u + 108 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 23 x 1 u
= 23 u
Total cost of books in the end
= 18 x (1 u + 6)
= 18 u + 108
23 u + 13 = 18 u + 108
23 u - 18 u = 108 - 13
5 u = 95
1 u = 95 ÷ 5 = 19
Number of books at first = 19
Answer(s): 19