The average cost of some books is $20.
When a book that costs $100 is added,
the average cost of the books becomes $22.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
20 |
22 |
Total value |
20 u |
22 u + 22 |
Number of books at first = 1 u
Number of books in the end = 1 u + 1
Total cost of the books at first
= 20 x 1 u
= 20 u
Total cost of books in the end
= 20 u + 100
Total cost of books in the end
= 22 x (1 u + 1)
= 22 u + 22
20 u + 100 = 22 u + 22
22 u - 20 u = 100 - 22
2 u = 78
1 u = 78 ÷ 2 = 39
Number of books in the end
= 1 u + 1
= 39 + 1
= 40
Answer(s): 40