The average cost of some books is $40.
When another 9 books are added,
the total cost increases by $100.
The average cost becomes $30.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 9 |
Value |
40 |
30 |
Total value |
40 u |
30 u + 270 |
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 40
= 40 u
Total cost of the books in the end
= 30 x (1 u + 9)
= 30 u + 270
40 u + 100 = 30 u + 270
40 u - 30 u = 270 - 100
10 u = 170
1 u = 170 ÷ 10 = 17
Number of books in the end
= 1 u + 9
= 17 + 9
= 26
Answer(s): 26