The average cost of some books is $27.
After 6 new books are added,
it adds another $17 to the total cost.
As a result, the average cost of all the books decreases to $22.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 6 |
Value |
27 |
22 |
Total value |
27 u |
22 u + 132 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 27 x 1 u
= 27 u
Total cost of books in the end
= 22 x (1 u + 6)
= 22 u + 132
27 u + 17 = 22 u + 132
27 u - 22 u = 132 - 17
5 u = 115
1 u = 115 ÷ 5 = 23
Number of books at first = 23
Answer(s): 23