The average cost of some books is $28.
After 3 new books are added,
it adds another $20 to the total cost.
As a result, the average cost of all the books decreases to $24.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
28 |
24 |
Total value |
28 u |
24 u + 72 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 28 x 1 u
= 28 u
Total cost of books in the end
= 24 x (1 u + 3)
= 24 u + 72
28 u + 20 = 24 u + 72
28 u - 24 u = 72 - 20
4 u = 52
1 u = 52 ÷ 4 = 13
Number of books at first = 13
Answer(s): 13