The average cost of some books is $60.
When a book that costs $84 is added,
the average cost of the books becomes $64.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
60 |
64 |
Total value |
60 u |
64 u + 64 |
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 + 84
Total cost of books in the end
= 64 x (1 u + 1)
= 64 u + 64
60 u + 84 = 64 u + 64
64 u - 60 u = 84 - 64
4 u = 20
1 u = 20 ÷ 4 = 5
Number of books in the end
= 1 u + 1
= 5 + 1
= 6
Answer(s): 6