The average cost of some books is $24.
When another 9 books are added,
the total cost increases by $120.
The average cost becomes $20.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 9 |
Value |
24 |
20 |
Total value |
24 u |
20 u + 180 |
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 24
= 24 u
Total cost of the books in the end
= 20 x (1 u + 9)
= 20 u + 180
24 u + 120 = 20 u + 180
24 u - 20 u = 180 - 120
4 u = 60
1 u = 60 ÷ 4 = 15
Number of books in the end
= 1 u + 9
= 15 + 9
= 24
Answer(s): 24