The average cost of some books is $21.
After 3 new books are added,
it adds another $19 to the total cost.
As a result, the average cost of all the books decreases to $17.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
21 |
17 |
Total value |
21 u |
17 u + 51 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 21 x 1 u
= 21 u
Total cost of books in the end
= 17 x (1 u + 3)
= 17 u + 51
21 u + 19 = 17 u + 51
21 u - 17 u = 51 - 19
4 u = 32
1 u = 32 ÷ 4 = 8
Number of books at first = 8
Answer(s): 8