The average cost of some books is $40.
When a book that costs $160 is removed,
the average cost of the books becomes $20.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u - 1 |
Value |
40 |
20 |
Total value |
40 u |
20 u - 20 |
Number of books at first = 1 u
Number of books in the end = 1 u - 1
Total cost of the books at first
= 40 x 1 u
= 40 u
Total cost of books in the end
= 40 u - 160
Total cost of books in the end
= 20 x (1 u - 1)
= 20 u - 20
40 u - 160 = 20 u - 20
40 u - 20 u = 160 - 20
20 u = 140
1 u = 140 ÷ 20 = 7
Number of books in the end
= 1 u - 1
= 7 - 1
= 6
Answer(s): 6