The average cost of some books is $52.
When another 9 books are added,
the total cost increases by $180.
The average cost becomes $40.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 9 |
Value |
52 |
40 |
Total value |
52 u |
40 u + 360 |
Number of books at first = 1 u
Number of books in the end = 1 u + 9
Total cost of books at first
= 1 u x 52
= 52 u
Total cost of the books in the end
= 40 x (1 u + 9)
= 40 u + 360
52 u + 180 = 40 u + 360
52 u - 40 u = 360 - 180
12 u = 180
1 u = 180 ÷ 12 = 15
Number of books in the end
= 1 u + 9
= 15 + 9
= 24
Answer(s): 24