The average cost of some books is $20.
After 4 new books are added,
it adds another $17 to the total cost.
As a result, the average cost of all the books decreases to $13.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 4 |
Value |
20 |
13 |
Total value |
20 u |
13 u + 52 |
Number of books at first = 1 u
Number of books in the end = 1 u + 4
Total cost of books at first
= 20 x 1 u
= 20 u
Total cost of books in the end
= 13 x (1 u + 4)
= 13 u + 52
20 u + 17 = 13 u + 52
20 u - 13 u = 52 - 17
7 u = 35
1 u = 35 ÷ 7 = 5
Number of books at first = 5
Answer(s): 5