The average cost of some books is $65.
When another 5 books are added,
the total cost increases by $210.
The average cost becomes $60.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 5 |
Value |
65 |
60 |
Total value |
65 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 65
= 65 u
Total cost of the books in the end
= 60 x (1 u + 5)
= 60 u + 300
65 u + 210 = 60 u + 300
65 u - 60 u = 300 - 210
5 u = 90
1 u = 90 ÷ 5 = 18
Number of books in the end
= 1 u + 5
= 18 + 5
= 23
Answer(s): 23