The average cost of some books is $21.
After 5 new books are added,
it adds another $10 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 |
21 |
16 |
Total value |
21 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
= 21 x 1 u
= 21 u
Total cost of books in the end
= 16 x (1 u + 5)
= 16 u + 80
21 u + 10 = 16 u + 80
21 u - 16 u = 80 - 10
5 u = 70
1 u = 70 ÷ 5 = 14
Number of books at first = 14
Answer(s): 14