The average cost of some books is $40.
When a book that costs $170 is removed,
the average cost of the books becomes $30.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u - 1 |
Value |
40 |
30 |
Total value |
40 u |
30 u - 30 |
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 - 170
Total cost of books in the end
= 30 x (1 u - 1)
= 30 u - 30
40 u - 170 = 30 u - 30
40 u - 30 u = 170 - 30
10 u = 140
1 u = 140 ÷ 10 = 14
Number of books in the end
= 1 u - 1
= 14 - 1
= 13
Answer(s): 13