The average cost of some books is $55.
When another 9 books are added,
the total cost increases by $340.
The average cost becomes $50.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 9 |
Value |
55 |
50 |
Total value |
55 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 55
= 55 u
Total cost of the books in the end
= 50 x (1 u + 9)
= 50 u + 450
55 u + 340 = 50 u + 450
55 u - 50 u = 450 - 340
5 u = 110
1 u = 110 ÷ 5 = 22
Number of books in the end
= 1 u + 9
= 22 + 9
= 31
Answer(s): 31