The average cost of some books is $50.
When a book that costs $90 is added,
the average cost of the books becomes $55.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
50 |
55 |
Total value |
50 u |
55 u + 55 |
Number of books at first = 1 u
Number of books in the end = 1 u + 1
Total cost of the books at first
= 50 x 1 u
= 50 u
Total cost of books in the end
= 50 u + 90
Total cost of books in the end
= 55 x (1 u + 1)
= 55 u + 55
50 u + 90 = 55 u + 55
55 u - 50 u = 90 - 55
5 u = 35
1 u = 35 ÷ 5 = 7
Number of books in the end
= 1 u + 1
= 7 + 1
= 8
Answer(s): 8