The average cost of some books is $80.
When another 9 books are added,
the total cost increases by $480.
The average cost becomes $70.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 9 |
Value |
80 |
70 |
Total value |
80 u |
70 u + 630 |
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
= 70 x (1 u + 9)
= 70 u + 630
80 u + 480 = 70 u + 630
80 u - 70 u = 630 - 480
10 u = 150
1 u = 150 ÷ 10 = 15
Number of books in the end
= 1 u + 9
= 15 + 9
= 24
Answer(s): 24