The average cost of some books is $71.
When another 6 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 + 6 |
Value |
71 |
60 |
Total value |
71 u |
60 u + 360 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 1 u x 71
= 71 u
Total cost of the books in the end
= 60 x (1 u + 6)
= 60 u + 360
71 u + 140 = 60 u + 360
71 u - 60 u = 360 - 140
11 u = 220
1 u = 220 ÷ 11 = 20
Number of books in the end
= 1 u + 6
= 20 + 6
= 26
Answer(s): 26