The average cost of some books is $70.
When another 9 books are added,
the total cost increases by $390.
The average cost becomes $60.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 9 |
Value |
70 |
60 |
Total value |
70 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 70
= 70 u
Total cost of the books in the end
= 60 x (1 u + 9)
= 60 u + 540
70 u + 390 = 60 u + 540
70 u - 60 u = 540 - 390
10 u = 150
1 u = 150 ÷ 10 = 15
Number of books in the end
= 1 u + 9
= 15 + 9
= 24
Answer(s): 24