The average cost of some books is $66.
When another 3 books are added,
the total cost increases by $120.
The average cost becomes $60.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
66 |
60 |
Total value |
66 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 66
= 66 u
Total cost of the books in the end
= 60 x (1 u + 3)
= 60 u + 180
66 u + 120 = 60 u + 180
66 u - 60 u = 180 - 120
6 u = 60
1 u = 60 ÷ 6 = 10
Number of books in the end
= 1 u + 3
= 10 + 3
= 13
Answer(s): 13