The average cost of some books is $30.
After 6 new books are added,
it adds another $18 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 + 6 |
Value |
30 |
24 |
Total value |
30 u |
24 u + 144 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 30 x 1 u
= 30 u
Total cost of books in the end
= 24 x (1 u + 6)
= 24 u + 144
30 u + 18 = 24 u + 144
30 u - 24 u = 144 - 18
6 u = 126
1 u = 126 ÷ 6 = 21
Number of books at first = 21
Answer(s): 21