The average cost of some books is $80.
When another 6 books are added,
the total cost increases by $100.
The average cost becomes $60.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 6 |
Value |
80 |
60 |
Total value |
80 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 80
= 80 u
Total cost of the books in the end
= 60 x (1 u + 6)
= 60 u + 360
80 u + 100 = 60 u + 360
80 u - 60 u = 360 - 100
20 u = 260
1 u = 260 ÷ 20 = 13
Number of books in the end
= 1 u + 6
= 13 + 6
= 19
Answer(s): 19