The average cost of some books is $24.
After 6 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 + 6 |
Value |
24 |
14 |
Total value |
24 u |
14 u + 84 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 24 x 1 u
= 24 u
Total cost of books in the end
= 14 x (1 u + 6)
= 14 u + 84
24 u + 14 = 14 u + 84
24 u - 14 u = 84 - 14
10 u = 70
1 u = 70 ÷ 10 = 7
Number of books at first = 7
Answer(s): 7