The average cost of some books is $80.
When another 7 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 + 7 |
Value |
80 |
60 |
Total value |
80 u |
60 u + 420 |
Number of books at first = 1 u
Number of books in the end = 1 u + 7
Total cost of books at first
= 1 u x 80
= 80 u
Total cost of the books in the end
= 60 x (1 u + 7)
= 60 u + 420
80 u + 140 = 60 u + 420
80 u - 60 u = 420 - 140
20 u = 280
1 u = 280 ÷ 20 = 14
Number of books in the end
= 1 u + 7
= 14 + 7
= 21
Answer(s): 21