The average cost of some books is $20.
When a book that costs $100 is added,
the average cost of the books becomes $25.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
20 |
25 |
Total value |
20 u |
25 u + 25 |
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
= 25 x (1 u + 1)
= 25 u + 25
20 u + 100 = 25 u + 25
25 u - 20 u = 100 - 25
5 u = 75
1 u = 75 ÷ 5 = 15
Number of books in the end
= 1 u + 1
= 15 + 1
= 16
Answer(s): 16