The average cost of some books is $30.
When a book that costs $80 is added,
the average cost of the books becomes $35.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
30 |
35 |
Total value |
30 u |
35 u + 35 |
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 + 80
Total cost of books in the end
= 35 x (1 u + 1)
= 35 u + 35
30 u + 80 = 35 u + 35
35 u - 30 u = 80 - 35
5 u = 45
1 u = 45 ÷ 5 = 9
Number of books in the end
= 1 u + 1
= 9 + 1
= 10
Answer(s): 10