The average cost of some books is $21.
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 $14.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 5 |
Value |
21 |
14 |
Total value |
21 u |
14 u + 70 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 21 x 1 u
= 21 u
Total cost of books in the end
= 14 x (1 u + 5)
= 14 u + 70
21 u + 14 = 14 u + 70
21 u - 14 u = 70 - 14
7 u = 56
1 u = 56 ÷ 7 = 8
Number of books at first = 8
Answer(s): 8