The average cost of some books is $20.
After 5 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 + 5 |
Value |
20 |
16 |
Total value |
20 u |
16 u + 80 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 20 x 1 u
= 20 u
Total cost of books in the end
= 16 x (1 u + 5)
= 16 u + 80
20 u + 16 = 16 u + 80
20 u - 16 u = 80 - 16
4 u = 64
1 u = 64 ÷ 4 = 16
Number of books at first = 16
Answer(s): 16