The average cost of some books is $40.
When a book that costs $85 is added,
the average cost of the books becomes $45.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
40 |
45 |
Total value |
40 u |
45 u + 45 |
Number of books at first = 1 u
Number of books in the end = 1 u + 1
Total cost of the books at first
= 40 x 1 u
= 40 u
Total cost of books in the end
= 40 u + 85
Total cost of books in the end
= 45 x (1 u + 1)
= 45 u + 45
40 u + 85 = 45 u + 45
45 u - 40 u = 85 - 45
5 u = 40
1 u = 40 ÷ 5 = 8
Number of books in the end
= 1 u + 1
= 8 + 1
= 9
Answer(s): 9