The average cost of some books is $70.
When a book that costs $190 is removed,
the average cost of the books becomes $50.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u - 1 |
Value |
70 |
50 |
Total value |
70 u |
50 u - 50 |
Number of books at first = 1 u
Number of books in the end = 1 u - 1
Total cost of the books at first
= 70 x 1 u
= 70 u
Total cost of books in the end
= 70 u - 190
Total cost of books in the end
= 50 x (1 u - 1)
= 50 u - 50
70 u - 190 = 50 u - 50
70 u - 50 u = 190 - 50
20 u = 140
1 u = 140 ÷ 20 = 7
Number of books in the end
= 1 u - 1
= 7 - 1
= 6
Answer(s): 6