The average cost of some books is $30.
When a book that costs $130 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 |
30 |
20 |
Total value |
30 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
= 30 x 1 u
= 30 u
Total cost of books in the end
= 30 u - 130
Total cost of books in the end
= 20 x (1 u - 1)
= 20 u - 20
30 u - 130 = 20 u - 20
30 u - 20 u = 130 - 20
10 u = 110
1 u = 110 ÷ 10 = 11
Number of books in the end
= 1 u - 1
= 11 - 1
= 10
Answer(s): 10