The average cost of some books is $70.
When a book that costs $91 is added,
the average cost of the books becomes $73.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
70 |
73 |
Total value |
70 u |
73 u + 73 |
Number of books at first = 1 u
Number of books in the end = 1 u + 1
Total cost of the books at first
= 70 x 1 u
= 70 u
Total cost of books in the end
= 70 u + 91
Total cost of books in the end
= 73 x (1 u + 1)
= 73 u + 73
70 u + 91 = 73 u + 73
73 u - 70 u = 91 - 73
3 u = 18
1 u = 18 ÷ 3 = 6
Number of books in the end
= 1 u + 1
= 6 + 1
= 7
Answer(s): 7