The average cost of some books is $26.
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 $21.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 5 |
Value |
26 |
21 |
Total value |
26 u |
21 u + 105 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 26 x 1 u
= 26 u
Total cost of books in the end
= 21 x (1 u + 5)
= 21 u + 105
26 u + 20 = 21 u + 105
26 u - 21 u = 105 - 20
5 u = 85
1 u = 85 ÷ 5 = 17
Number of books at first = 17
Answer(s): 17