The average cost of some books is $28.
After 5 new books are added,
it adds another $14 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 |
28 |
21 |
Total value |
28 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
= 28 x 1 u
= 28 u
Total cost of books in the end
= 21 x (1 u + 5)
= 21 u + 105
28 u + 14 = 21 u + 105
28 u - 21 u = 105 - 14
7 u = 91
1 u = 91 ÷ 7 = 13
Number of books at first = 13
Answer(s): 13