The average cost of some books is $50.
When a book that costs $170 is removed,
the average cost of the books becomes $30.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u - 1 |
Value |
50 |
30 |
Total value |
50 u |
30 u - 30 |
Number of books at first = 1 u
Number of books in the end = 1 u - 1
Total cost of the books at first
= 50 x 1 u
= 50 u
Total cost of books in the end
= 50 u - 170
Total cost of books in the end
= 30 x (1 u - 1)
= 30 u - 30
50 u - 170 = 30 u - 30
50 u - 30 u = 170 - 30
20 u = 140
1 u = 140 ÷ 20 = 7
Number of books in the end
= 1 u - 1
= 7 - 1
= 6
Answer(s): 6