The average cost of some books is $29.
After 3 new books are added,
it adds another $15 to the total cost.
As a result, the average cost of all the books decreases to $20.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
29 |
20 |
Total value |
29 u |
20 u + 60 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 29 x 1 u
= 29 u
Total cost of books in the end
= 20 x (1 u + 3)
= 20 u + 60
29 u + 15 = 20 u + 60
29 u - 20 u = 60 - 15
9 u = 45
1 u = 45 ÷ 9 = 5
Number of books at first = 5
Answer(s): 5