The average cost of some books is $30.
When a book that costs $90 is added,
the average cost of the books becomes $32.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
30 |
32 |
Total value |
30 u |
32 u + 32 |
Number of books at first = 1 u
Number of books in the end = 1 u + 1
Total cost of the books at first
= 30 x 1 u
= 30 u
Total cost of books in the end
= 30 u + 90
Total cost of books in the end
= 32 x (1 u + 1)
= 32 u + 32
30 u + 90 = 32 u + 32
32 u - 30 u = 90 - 32
2 u = 58
1 u = 58 ÷ 2 = 29
Number of books in the end
= 1 u + 1
= 29 + 1
= 30
Answer(s): 30