The average cost of some books is $66.
When another 5 books are added,
the total cost increases by $240.
The average cost becomes $60.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 5 |
Value |
66 |
60 |
Total value |
66 u |
60 u + 300 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 1 u x 66
= 66 u
Total cost of the books in the end
= 60 x (1 u + 5)
= 60 u + 300
66 u + 240 = 60 u + 300
66 u - 60 u = 300 - 240
6 u = 60
1 u = 60 ÷ 6 = 10
Number of books in the end
= 1 u + 5
= 10 + 5
= 15
Answer(s): 15