The average cost of some books is $60.
When a book that costs $200 is removed,
the average cost of the books becomes $40.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u - 1 |
Value |
60 |
40 |
Total value |
60 u |
40 u - 40 |
Number of books at first = 1 u
Number of books in the end = 1 u - 1
Total cost of the books at first
= 60 x 1 u
= 60 u
Total cost of books in the end
= 60 u - 200
Total cost of books in the end
= 40 x (1 u - 1)
= 40 u - 40
60 u - 200 = 40 u - 40
60 u - 40 u = 200 - 40
20 u = 160
1 u = 160 ÷ 20 = 8
Number of books in the end
= 1 u - 1
= 8 - 1
= 7
Answer(s): 7