The average cost of some books is $30.
When a book that costs $150 is removed,
the average cost of the books becomes $10.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u - 1 |
Value |
30 |
10 |
Total value |
30 u |
10 u - 10 |
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 - 150
Total cost of books in the end
= 10 x (1 u - 1)
= 10 u - 10
30 u - 150 = 10 u - 10
30 u - 10 u = 150 - 10
20 u = 140
1 u = 140 ÷ 20 = 7
Number of books in the end
= 1 u - 1
= 7 - 1
= 6
Answer(s): 6