The average cost of some books is $80.
When another 9 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 + 9 |
Value |
80 |
60 |
Total value |
80 u |
60 u + 540 |
Number of books at first = 1 u
Number of books in the end = 1 u + 9
Total cost of books at first
= 1 u x 80
= 80 u
Total cost of the books in the end
= 60 x (1 u + 9)
= 60 u + 540
80 u + 120 = 60 u + 540
80 u - 60 u = 540 - 120
20 u = 420
1 u = 420 ÷ 20 = 21
Number of books in the end
= 1 u + 9
= 21 + 9
= 30
Answer(s): 30