The average cost of some books is $20.
After 6 new books are added,
it adds another $16 to the total cost.
As a result, the average cost of all the books decreases to $16.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 6 |
Value |
20 |
16 |
Total value |
20 u |
16 u + 96 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 20 x 1 u
= 20 u
Total cost of books in the end
= 16 x (1 u + 6)
= 16 u + 96
20 u + 16 = 16 u + 96
20 u - 16 u = 96 - 16
4 u = 80
1 u = 80 ÷ 4 = 20
Number of books at first = 20
Answer(s): 20