The average cost of some books is $64.
When another 3 books are added,
the total cost increases by $140.
The average cost becomes $60.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
64 |
60 |
Total value |
64 u |
60 u + 180 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 1 u x 64
= 64 u
Total cost of the books in the end
= 60 x (1 u + 3)
= 60 u + 180
64 u + 140 = 60 u + 180
64 u - 60 u = 180 - 140
4 u = 40
1 u = 40 ÷ 4 = 10
Number of books in the end
= 1 u + 3
= 10 + 3
= 13
Answer(s): 13