The average cost of some books is $59.
When another 9 books are added,
the total cost increases by $270.
The average cost becomes $50.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 9 |
Value |
59 |
50 |
Total value |
59 u |
50 u + 450 |
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 59
= 59 u
Total cost of the books in the end
= 50 x (1 u + 9)
= 50 u + 450
59 u + 270 = 50 u + 450
59 u - 50 u = 450 - 270
9 u = 180
1 u = 180 ÷ 9 = 20
Number of books in the end
= 1 u + 9
= 20 + 9
= 29
Answer(s): 29