The average cost of some books is $60.
When a book that costs $100 is added,
the average cost of the books becomes $65.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
60 |
65 |
Total value |
60 u |
65 u + 65 |
Number of books at first = 1 u
Number of books in the end = 1 u + 1
Total cost of the books at first
= 60 x 1 u
= 60 u
Total cost of books in the end
= 60 u + 100
Total cost of books in the end
= 65 x (1 u + 1)
= 65 u + 65
60 u + 100 = 65 u + 65
65 u - 60 u = 100 - 65
5 u = 35
1 u = 35 ÷ 5 = 7
Number of books in the end
= 1 u + 1
= 7 + 1
= 8
Answer(s): 8