The average cost of some books is $40.
When a book that costs $70 is added,
the average cost of the books becomes $43.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
40 |
43 |
Total value |
40 u |
43 u + 43 |
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 + 70
Total cost of books in the end
= 43 x (1 u + 1)
= 43 u + 43
40 u + 70 = 43 u + 43
43 u - 40 u = 70 - 43
3 u = 27
1 u = 27 ÷ 3 = 9
Number of books in the end
= 1 u + 1
= 9 + 1
= 10
Answer(s): 10